"Sir, we've just had a whole cargo-ship full of whoop-ass dumped on us! We ran into a new strain of Ultralisk and it took a lot o' pepper to bring it down. To top it all off, our recon squad reports that the critter's been reincarnated by a nearby cerebrate and is on its way back for more!"
A torrasque appears in StarCraft: Enslavers in the final mission of the A-branch missions, where the player rescues the protoss forces. The torrasque will attack the player's base and respawns some time later when killed. Until the cerebrate is killed, the torrasque will forever reincarnate. This same mechanic is used in Episode V in "To Chain the Beast," where the torrasque is spawned by the brown-colored zerg force until their cerebrate is killed.
In both missions, walling off the player's base with supply depots is the best way to defend against the torrasque. Several siege tanks in siege mode and battlecruisers with the Yamato cannon are best to bring it down, as its high armor makes it highly resistant to weaker units.
A torrasque can be found amongst the feral zerg in the Episode VI mission Reign of Fire, and can be recruited to the player's control.