Hydralisk den

The zerg hydralisk den bears only a passing resemblance to the nests of the once peaceful slothien that the hydralisks evolved from. Containing hydralisk genetics, the structure allows larvae to morph into hydralisks.

StarCraft
The hydralisk den can also provide the genetic code necessary for Muscular Augments (faster movement), Grooved Spines (longer range) and the Lurker Aspect (allows hydralisks to morph into lurkers).

StarCraft II
The hydralisk den appears in StarCraft II and unlocks the hydralisk.

Development
The hydralisk den could transform into a deep warren/lurker den, enabling hydralisks to transform into lurkers.