There've been a few. One of the most notorious was Emperor Hawthorne III., who lived twelve-hundred odd years ago. (9th century a.i.) He expanded the penal colony program of his predecessors and exiled people on an unprecedented scale. Non-human sophonts like nephilim and goblins were deported indiscriminately; humans were exiled for being political dissidents, intellectuals, gay, or any of dozens of other "crimes against the imperial order". He was eventually assassinated by rebels.
Some previous emperors were even more genocidal, if less fascist. Ironclad VI., who lived eighteen-hundred years ago (3rd century a.i.), advanced a policy of complete genocide against all non-human sophonts. Wherever the expansionist Empire clashed with indigenous nephilim, giants, troglodytes or goblins, the legions were sent to utterly exterminate them.
In more recent history, we had Empress Nahema Stark I. (15th century a.i.), under whose expansionist ambitions the Empire attempted to colonize further reaches of the caves. The civilizations that lived below (chiefly the Vahnatai and the Slithzerikai, as well as other, even remoter denizens of the abyss) perceived the threat and formed the Deep Alliance to halt the Empire's intrusion. The Shard War that followed (so named for the devastating use of crystallothaumic weapons of mass destruction, or crystal shards) was deadlier than any conflict in history for raw body count (though not relative to population).
Counting Nahema Stark I. among these leaders is sometimes contested because she advanced a progressive agenda domestically. However, her militarist policy led to millions of deaths on both sides. It is estimated that about 800,000 Imperial soldiers and 600,000 Alliance soldiers perished, and more than two million civilians were killed directly; more than three million as a result of famine and disease in the parts of the caverns that were cut off by the war.
