The Cavern of Many Deaths was a cavern underneath Kitalpha. It allowed you to go under the city and not miss a thing of what was going on above.
Menkar was slain by Queen Alrescha in the Cavern, and it was used as an escape route for Rigel Squirrelking and Hollyfire Squirrelqueen. Later, Arietis used it, however she branched off the main path, and ended up at the end of one of the Cavern's halls, towards the south end of Mossflower Woods.
Later, Kyo Firesly's vermin patrol, headed by Captain Lag, accidentally dug through into the Cavern of Many Deaths, and eventually dug into the ruins of Kitalpha. However, Morning, berserk and nearly dead, rushed at Lag's patrol, and killed every one of them (Morning's final stand).
Soon after, an earthquake hits, and Lag's tunnel collapses.
Somehow, Reedwater, Stonedusk and Rigel manage to find the Cavern, and bury Hollyfire not far from where Menkar was buried.