Blobs are rather common monsters and appear in a multitude of colors. While not as ubiquitous as skeletons, orcs and goblins, there is no seasoned high fantasy PC that has not faced a blob once or twice already. I wonder if H.P Lovecraft is to blame for those creatures or if there was a blob monster before the Shoggoth in his novella At the Mountains of Madness.
What follows is another take of mine on the blob concept: designed as an obstacle to be placed into a dungeon. My players will encounter it when they return to a certain dungeon: one of the pit-traps they found will now contain this monster, ready to go after them once they jumped over it. Most players know how to deal with creatures that don´t seem to be vulnerable to their regular means of attack. The tried-and-true response of a seasoned gamer will be fire. By creating this blob, I plead myself guilty of being a dick.
The creature fills the air with a stinging, acrid stench that characters may smell from yards away, and it will worsen the closer they get. It looks like the typical blob, with an unhealthy gale-yellow color. The stats provided fit my OSR style:
Armor Class: 10
Hit Dice: 2 (20 hit points)
Attacks: 2 (contact)
Damage: 1d6 on contact; 1d4 on the following round; 1 point the round thereafter
The creature attacks by touch, it secretions are acidic and cause continuing damage (see above). Any attempt to clean the wound with water will double the damage on the next round, but alcohol will neutralize it. If two attacks in a row where successful against the same target, the blob begins to “engulf” it. This means an automatic hit on the next and all following rounds (until destroyed or forced to retreat): in this case 3d6 are rolled for damage and the highest result is kept. At the end of a fight, there is a chance that organic armor my have its AC reduced by one step (x out of 8 chance, where x is the number of successful attacks against the character; exchange the “8” with a “6” if the character had been engulfed at least once during the fight).
The blob is immune against regular attacks, but vulnerable to magic and fire alike. In fact, actually getting harmed by anything is so alien to the creature that it must check Moral at the beginning of the own turn if it received any damage before. On a failure, it will retreat. A turn later the blob will already have forgot about the fight and may be encountered again (wandering monster): the creature has no real memory.
While fire is an effective weapon, it is a double-edged sword: burning the blob produces poisonous fumes. Every character within 30´ of the blob must pass a Saving Throw vs. Poison at the beginning of the following round or is going to suffer half of the damage that the blob has suffered from the fire. The toxin goes from the lunges directly into the blood and destroys blood cells (the victim grows pale and feels weak). It is up to the GM if this fumes linger, and for how many rounds).