Shadowstalkers

The Shadowstalkers are spies — whose job description includes assassinations — and are in Northrend by orders of Varimathras. For instance, Shadowstalker Luther was gathering intelligence on some nearby nerubians before he ran into necrolords and had to poison himself rather than be taken as a prisoner. Subsequently, they were fighting the Nerub'ar who were turning Horde soldiers into undead. Shadowstalker Barthus seems to be in charge of the Shadowstalkers in the area.

Speculation
They may be a branch of the Deathstalkers.