Spectre

A Spectre (in American, Specter) is the Undead spirit which haunts those bourgeoisie as haven't yet accepted the reality of class-dialectic and the inevitability of Communism.

In Dungeons & Dragons, the Spectre always takes the European spelling. This is just a powerful Wraith with twice the energy-drain. As late as 1977, Eric Holmes acknowledged that this was the ring-wraith, the "Nazgul of Tolkien" - so, like the Balrog, the lawyers told TSR to file off those serials. If a Spectre kills you, you become a weakened Spectre yourself - heavily implied, that your undead spirit takes Wraith stats.

It arose in the first Monster Manual and the Eric Holmes 1977 box-set: the one bundled with B1, rather than with B2. In those ancient days, Holmes implied you could turn them on an 11-12 roll on 2d6 (=1 in 18, yay math) at fifth level; AD&D's "Matrix for Clerics Affecting Undead" (DMG matrix "III.") agreed, but boosting to 20 on a d20.

The Spectre will fuck up your party at 6th level, so one of them is a boss monster in X3: Curse of Xanathon. At higher level, the Restoration spell takes the edge off. At 6th (rather, 4th) the local priests can probably hook you up if you'd got the damage in the course of doing good - completing X3 assuredly counts.

How a Spectre doesn't just sail into town one night and make a Wraithopolis under his command is a problem that must be solved by any worldbuilder. In the Known World of the Expert series, that is more-or-less what one Spectre crew sets out to DO in one setpiece of X10.