a special hard event that spawns a singular slime that will move towards nearest living creature and try to kill it - if it kills it it makes a new slime that follows the same procedure..
it will continue untill it reaches a certain limit and then leaves the map migrating onwards.
(edit - to Mod suggestion)
if code permits make basic projectile weapons deal alot less dmg and fire/explosives are needed for "effective" clearing.
Have you played minecraft, you can find a similar mob there :P jokes :)
ok then i just need to have all of the rimworld gameplay added to minecraft :D
....actually that would be kinda boss