Do you know what "nemesis" means? A righteous infliction of retribution manifested by an appropriate agent. Personified in this case by an 'orrible bear... me.
From the announcement:
Fantasy Flight Games is proud to announce Nemesis, an expansion for the Relic board game. As the war in the warp-rift torn Antian Sector rages on, deadly new threats to the Imperium converge, drawn by the dark powers of Chaos. These new Nemeses are eager to carve out a place for themselves, dislodge the Imperium’s hold on the sector, and crush the Emperor’s agents once and for all.
For the first time, Nemesis introduces player versus player combat in an optional new Enemies of the Imperium game mode. Enemies of the Imperium allows up to two players to play as Nemeses instead of characters, such as a World Eaters Beserker, or a Eldar Dire Avenger.
Nemesis players work to gain infamy by defeating agents of the Imperium, and resolving engagements throughout the Sector. As each side vies for control of the Antian Sector, Nemesis players and Imperium characters can clash in battle. By gaining enough infamy, Nemesis players can prevent the Imperium from resolving the scenario in the Inner Tier, delivering a mighty blow to the Imperium’s presence in the Antian Sector and winning the game.
The Enemies of the Imperium game mode isn’t the only way players can oppose each other. The Nemesis expansion also adds player versus player combat through a standalone Apostates versus Devotee mechanic. Nemesis introduces new Apostate wargear, proscribed technology created by enemies of the Imperium. Characters who choose to try to use these weapons become Apostates, subject to the prosecution of other, more puritanical, Devotee characters.