You receive Calmseas' Blessed Salt with unlimited charges at the beginning of the quest.
To start The Battle, wander around Kithicor Forest looking for wandering skeletons, and kill them. I have found wandering skeletons as far North as The Blessed Knoll, East as the Ivy Retched Remnants, and West at the Stillwater Bridge (so quite a large radius). Reports differ as to the number required to start the battle (I had to kill 6), 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 ) Copy/waypoint 483, 55, 1001
To do this, kill the dark elf spirits. A defeated Teir'Dal spirit spawns when each dies. Use the salt on the defeated Teir'Dal spirit.
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).