The very rare slimes were dark green in color, and had large sludgy bodies.
The oozes attacked by poisoning foes, as well as magically slowing them. After their foes retaliated, the slimes would split, forming new slimes, which would also engage in combat. The new slimes could also be split further, resulting in a large amount of them spawning, if their foes continued to attack them. They would only cease to multiply, and die, when exposed to fire.
In 1368 DR, fission slimes were known to inhabit Durlag's Tower. The slimy creatures were also mentioned in Elminster's ecology guide, Elminster's Ecologies Appendix IIIa. Here, the Sage of Shadowdale wrote that they would always split into new, independent creatures when attacked, unless they were set aflame.