Not available in single player. Lasombra enemies in single player use Dark Hunter, the only skill granted by this tome.
Players cannot choose a Lasombra character in multiplayer without a mod. Lasombra enemies in multiplayer can use Dominate and Potence skills as well as Obtenebration.
The Lasombra clan and powers were added by the Within the Darkness mod, but the powers were still reserved in comparison to the Mind's Eye / tabletop version.
Presumably the greater power of the tabletop and Live Action Roleplaying Lasombra were to give the Game Masters an edge for their NPCs