Since it's inception the world of Dungeons & Dragons and it's simulacra has been filled with all manner of vile oozes, slimes and jellies. However, I have found, that while these are all deadly in their own right, none quite imitate or replicate my favorite jelly of all, the Blob. For statistics, I will be basing the creature on the original 1958 film, using Swords & Wizardry as my system of choice.
HD Half-scalable; AC 5; Atk Slam (1d4 scalable); Move 3; Save 18 scalable; AL N; CL/XP 5/240 scalable; Special: dissolve after hit, automatically latches on to target after hit, absorbs target after death, immune to all damage except cold
Sages believe that blobs may hail from another star, having traveled the cosmos by meteor. Whether this is their origin or not, is uncertain. Blobs are, however, often found in locations where meteors have touched down.
Blobs appear in many sizes and look like large globs of translucent red to purple jelly. Blobs move slowly by sliding across surfaces. A blob is amorphous and can squeeze through nearly any crack or crevice, regardless of the blob's size. A blog can even split itself to ooze through multiple passages at once, so long as it rejoins itself on the other side.
Blobs begin life as small insignificant things. They are, however to be feared. The smallest blobs begin no larger than an apple. When a blob hits a target, the target must make a saving throw with a penalty of the damage dealt. If the target fails the save, the blob latches on and begins to absorb the target, automatically dealing it's damage every round, growing larger with every round, until the target is wholly consumed. The blob gains mass equal to each of it's victims, as well as gaining the victims hit dice. Meaning, if a 1HD blob consumes a level 2 fighter, it will grow by the size of the fighter and become a 3HD monster. All statistics dependent upon HD will increase according to the blobs new HD along with the blob's CL/XP. For every 3 HD a blob has, it's damage will increase accordingly: >1HD-2HD 1d4, 3HD 1d6, 6HD 1d8, 9HD 1d10, 12HD 2d6, 15HD 3d6, 18HD 5d4, 21+HD 7d4
In truth there is no known way to kill a blob. Indeed, blobs are immune to all damage, save that of cold. In fact, the cold damage will not kill a blob outright. However, when a blob is exposed to extreme cold, it will be frozen solid. If a blob take it's hit points in cold damage, it will likewise, be frozen. A blob will do all it can to flee from cold.