It can be played by any chaos-sided player and its profile is:
Dark Naga
Monster
4 att.
4 Hits
4+ Svg
0-1 min/max
90pts
Spécials:
Ghost Sword: ignore save
Soul Drinker: cause Terror
Snake Body: always touch ennemy upon 4+, can enter any terrain Infantry can
If joining a undead army, the Dark Naga becomes a Naga Specter and get the same rules as any undead with unmodified stats.
Summarized Fluf:
After Be'lakor discovered Nagash secrets and learned to drink souls he merged with its dark magic souls and Ind monsters created Dark Nagas. Dark Nagas are soul drinkers, killing to release souls and fuel their master's power.