Fission Slimes are a rare form of slime. Like all slimes they are immune to normal weapons and backstab. They are also immune to electricity and piercing damage. They are resistant to missiles and cold and vulnerable to fire, acid, and magic. They have the ability to poison and slow their enemies from a distance and at close range.
An easy way to deal with them, will be to cast Doom to lower the slime's saving throws and then using a Wand of Polymorphing to turn it into a squirrel. This bypass the need for them to die by "fire attacks". Although the polymorphed squirrel's death animation shows it splitting into 2 slimes, the party will almost immediately be teleported into the next part of the Durlag's Tower challenge.
- It should be noted that they have to die from a fire-based attack, either magical or non-magical (by arrows), or they will split into two more slimes. If the split bugs, i.e. not dying to fire, or if one is using the "cheat keys" namely CTRL-Y, you will be bugged in Durlag's Tower and cannot advance without the use of a script. More info can be found on that particular bug here.