Mr. Lavant is covered in custard.

"Mr. Lavant is covered in custard.
Or perhaps it's some form of slime mold. Custard as rule doesn't move on its' own. Light your torches boys. Don't worry about Lavant it's too late for him. He would have wanted it this way, poor, brave, old chap."

Slime molds are about the most interesting and naturally creepy things in nature. As such they have earned a bit of place in Old D&D and fantasy games in general. Large magically mobile and aggressive molds, puddings and slimes are splattered all over the monster manuals.

I want to step back from the fantasy for a second and look at these nasty buggers as they are in nature. Then let's take that information and use it build some slimes for our games.

Source

First off Slimes in many fantasy games are considered dungeon or cave monsters, and that makes sense. Damp and devoid of light a typical dungeon setting would be a great place for a huge slime to thrive. As long as there's food around. A constructed and maintained underground structure might have the right climate but the wrong amount of ground litter to support a huge Slime. Slimes need to stay damp and eat. In the natural world eating means engulfing and digesting bacteria on decomposing plants. In our fantasy dungeons, our bigger meaner slimes are looking for more exotic prey, like player characters. IN these cases the creature will attempt  to attack restrain and engulf characters so that they can be digested like any other bacterium.

For me the key to fantasy game slimes should be overall size and pseudopodia or false feet. These are the veiny amoeba like appendages the slime will extend into it's environment looking for food. Why are tehy key ... They are exceptionally creepy.

"Your lantern throws a powerful spotlight into the vaulted chamber. The old mine has seen better days, puddles glisten among fallen timbers, running water can be heard somewhere to your left. The smell of rotting flesh is nearly overwhelming, your lantern finds the source, the skeletal remains of an unfortunate burrow outlined in the gloom near the room's center."

Standard setup, add that the dead burrow has been digested by one of our giant slime molds and the floor is a network of  pseudopodia just waiting to detect the next warm blooded creature through the door. The players could detect them by checking the floor carefully, or by creating a more wide spread light source than their hooded lantern. Just seeing the long tube like appendages wouldn't give the players any indication of what they are or what their purpose could be. Slime mold amoebae only aggregate when food is getting scarce, if a mold is encountered in this form it will be actively seeking something to eat.

• Allow the  pseudopodia to attack for low damage.
• Give them each  about  1/10th of the  hit points you are allowing for the main slime.
• If a player gets hit twice, have them make a strength check (or a save) or be  grappled.
• The main slime attacks grappled characters.
• The main slime may attack multiple targets on the same round
• When the main slime attacks use all of the monster manual slime effects.
• Killing  pseudopodia does not affect the main slime.
• Severed  pseudopodia can form their own slime colonies over time. In our example they will hit the floor with a splat and  appear to be "dead." As single cells that have aggregated together slicing them apart with a sword won't get it done. With that said the severed  pseudopodia aren't large or organized enough to pose a threat.
• Fire is the  only way to truly a slime as it kills the cells.
• A giant slime mold will retreat from natural sunlight or spell effects that specifically claim to mimic natural sunlight.  pseudopodia will retreat first back into the  main body of the creature then the main creature will attempt to retreat.
• Slimes caught in sunlight light will dry out. It will take time this thing is big I woudl suggest 1d10 damage per round for most games, use your discretion..
• Once dry the slime will stop moving and go "reproductive" (see below*)
• It is possible for a party to stumble on a slime mold that has already  gone into a reproductive state due to no finding enough food.

*When a slime mold goes into its reproductive cycle some of the cells that make up the mass of the "creature" form into stalks that lift up the other spells that morph into clusters of spores. These spores if disturbed drift in the wind and on the fur of clothing of animals until they find enough moisture to hatch into amoeba and start the life cycle over again. This is where small is dangerous. Player characters who disturb the  dried out form of a giant slime mold, will either carry the  mold on their  persons or worse yet breath the spores in and become a host.

Here we again step away from the real world and into fantasy. A huge slime mold would take quite a bit of time form, however if the  party is exposed to a giant slime mold's spores, having one erupt off the fur of a warriors cape when they bed down in a damp area would be great fun.

Breathing in the spores is bad news. I know some slimes and puddings were fatal in Old D&D, which makes sense to me.

Rather than directly fatal I would have the character get ungodly sick. Like the worst sinus infection ever. Perhaps after a while have some of the sickly yellowish pseudopodia try to exit the poor character at night. It would take a powerful cleric or Druid to help the  character.

Give the player a constitution check or equivalent daily (depending on what game you are running.)

Each time the character fails a save move to the next stage.

• Stage 1: Fever headache  (- 2 constitution temporary.)
• Stage 2: No energy, headache increases, Must make a concentration check to cast spells. (- 4 constitution -2 wisdom both temporary.)
• Stage 3: The pseudopodia might be visible at night 20% chance, dizziness, lethargy can't cast spells. - 6 constitution.
•  Stage 4: Can't walk, mutter incoherently, 40% chance of a visible pseudopodia at night. (- 2 constitution permanently per day.)
• Stage 5: Coma state (- 4 Constitution permanently per day.) When Constitution hits zero death occurs. The day after death a new giant slime mold will erupt from the hollowed out victim looking for more food.

Cures:

1. A druid will identify this malady with a normal check, if not automatically.
2. A druid can stop the  disease's progression at it's current stage and  cure it in 1d6 days wiht a healing check. 
3. If  the Druid happens to be treating a character in stage 4 or 5 of the disease, the patient will still lose 1 constitution permanently each day of the treatment if in stage 4, and 2 points of constitution per day if in stage 5.
4. A cleric's Cure disease spell or other magical equivalent will stop the progression of the disease and cure the ailing character, though the characters involved never know what the root cause of the  disease was.
5. A Ranger has a 5% chance per level of identifying the disease, and may use the heal skill to stop it's progression. A ranger will not have the ability to cure the disease however, and will have to seek a greater healing power.

Hydnellum peckii (Its a fungus not a slime but kind of great on it's own)

Rumors, myths, and wives tales about slimes.
The following items are things characters might hear about Simes if they happen to ask around about such things. Some of these rumors are true some are not, which ones are which is completely up the the GM.

1. An old woman in town says "Once dried then Slime mold is edible and  gives a man back the vigor that he may have lost with age."
2. Parents tell their children, "An active slime mold caught out during a full moon will form a ring. Within the gelatinous circle is an area strongly connected to the fey."
3. Some travelers have said, "Any part of a slime no matter how small will eventually grow into a new full flged slime."
4. It is widely believed, "Giant slimes are created when lightning strikes a ferry circle."
5. The old fisherman once said, "Giant slimes can exist perfectly well underwater, in fact a water born slime can grow to incredible sizes."
6. It is said, "If a giant slime grows in a container it will hold the shape of that container."
7. One caravan guard once told me "A slime can get into wood and sometimes even animate it, in fact mimics are just slime colonized chests."
8. An old man told me "Slimes can lay dormant for years, decades even  just waiting for blood."
9. Some rangers believe, "If a giant slime ingests local fungus the slime takes on any beneficial or toxic properties of that fungus."
10. "Sometimes a slime will pick up metal objects that they can not digest and incorporate them into their  bodies." At least that's what Noran the traveler says.
11. In one of  the wizard Grilacks papers it was written, "Some slimes hibernate in a hard shell formed by drying of it's outer layers. If disturbed eruptive pseudopodia will violently explode from any crack in the shell in a crude attempt at ambushing prey."
12. Nathers wife says "Slime dust is repelled by boiled urine."
13. In the  library at Torin a scroll records the largest slime ever seen. It describes it as, "After a week of rain it rose for the depths of the lake.Yellow and green in colour and  no more than a foot thick it covered the whole of the lake near Aslrihn. It's tentacles extended hundreds of years into the  town  and  killed several fishermen, live stock, and others before it was finally put to the torch. At that size it must have been ancient, however exactly how long it had been lurking there in the deep no one can tell"
14. The old wizard once told his students, "There are over seventy different species of  slimes, or sometimes called puddings, each different in aggression, appearance, and predilection to do harm. So no, one grey ooze is not the same as any other."
15. It is widely held that extreme cold stops a slime in it's tracks but  can not kill it.
16. It is widely known, Spores of slime the mold can fetch a good price per ounce from the  right alchemist, wizard, or other patron.
17. An old wives tale says, "A mixture of dried giant slime, cats urine boiled three times, and ground bone dust, left in a covered jug  in a dark place will over time create a vivid blue dye for  fabrics. The same recipe with the addition of ground brimstone and left in the sunlight will create a soft red dye."
18. Legend has it, "If a giant slime mold engulfs a sleeping child it will give rise to a goblin prince."
19. Giants: Hill giants consider giant slimes a delicacy. They will boil them to eat as a thick soup. The resulting sludge like broth is incredibly poisonous to most humanoids. Stone giants will often nurture and feed large slime mold specimens which they will then drape over their shoulders before they go into battle. Stone giants have even been known to hurl slimes at intruders rather than the more typical boulder. Fire giants abhore giant slime molds and will kill them on site.
20. Some say "Not all slime molds are mindless eukaryotic organisms. Some are single creatures ancinet and wise. No One knows if this is simply a magic fueled evolution of the slime mold or simply a case of two unrelated creatures sharing a similar appearance.

Thank you for reading.
-Mark.

Some sources:
http://www.slideshare.net/carmody60311/protista-example-organisms
http://viroola.com/2014/05/18/bleeding-tooth/
http://www.biologyreference.com/Se-T/Slime-Molds.html
http://fungus.org.uk/nwfg/funmay98.htm