This is a mod I made recently that adds a new miniboss enemy, the Death Knight.
Some screenshots:
The exterior of a Spire. Death Knights are found at the tops of these. These only generate when you are at least 35 chunks from the origin (this number is, as of 1_02, configurable).
A Spire's treasure room (as it appears in 1_02), located at the base of the Spire's central section. There is loot in that chest.
A Death Knight. These baddies have wither-skeleton like powers, but they look scarier, have more health (the exact amount is configurable), and can dish out a lot more damage.
He is standing next to a glowstone lantern.
A Golden Death Knight. This version has slightly more health still (again, the exact amount is configurable), and is quite a bit rarer (defaults to 95 per thousand Death Knights; this is configurable), but drops better stuff. There is also a spider in the background.
Download version 1.01
Download version 1.02 (changelog)