@deutschZuid is right on.
In Diablo II, Rakanishu is a unique Carver mob that always spawns next to the Cairn Stones in Stony Field.
Rakanishu is a Super Unique Carver found in the Stony Field next to the Cairn Stones that activate the portal to Tristram during the Search for Cain quest.
It is also said by event mobs in Diablo III. An "Avatar of Rakanishu" makes a cameo in the Dahlgur Oasis and drops a unique named weapon.
There is a randomized event in the Dalghur Oasis that is tied to Rakanishu in Diablo III as well. Several Fallen members on top of a hill are seen performing a ritual at a stone surrounded by 5 other stones. Upon destruction of the 5 stones, an Avatar of Rakanishu arrives. Killing him prompts the optional objective to destroy the stone he arrived at, before the event is completed. The 5 stones surrounding the center are also a nod to the Cairn Stones from Act I of Diablo II. The title shown when targeting the Avatar of Rakanishu is 'Fallen Demigod'. This is most likely why the Fallen Ones sometimes utter his name as a war cry.