To start The Battle, wander around Kithicor Forest looking for wandering skeletons, and kill them. Reports differ as to the number required to start the battle, but it appears that 1) all the wandering skeletons must be killed to start The Battle, and 2) wandering skeletons don't respawn when killed until after The Battle.
Apply blessed salt to the souls of twelve slain Teir'Dal ( 483, 55, 1001 ) /waypoint 483, 55, 1001
NOTE: Be selective in which spirits you target. Attacking elite dark elf spirits, or worse, the named ghosts, will cause all of them to attack you. Attacking a single spirit in the midst of a bunch of nearby elites can do the same. It is actually possible to survive a mass attack (see the discussion page), but most likely you will be invited to a dirt nap, courtesy of the whole gang.
NOTE 2: I found that attacking dark elf spirits that are not already engaged in combat prevents the 'mob attack' (disable AOE effects, lest you upset the mob).