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The Dark Foci are 3 foothold-type mini-bosses scattered throughout the River of Souls. As with all footholds, this one spawns elites and lesser trash.
In the case of the Dark Foci, the elites to worry about are Force of Corruption. These spawn about every minute, and should be focused down first. Do not allow the Focus to die while any of these are up, as the Force of Corruption will explode, causing your raid to suffer an after-win wipe.
At the same time as the Forces of Corruption, Corrupted Souls will spawn. These can be easily tanked, and shouldn't require much AoE do to an ability of the focus itself.
The Dark Focus casts 3 spells throughout the encounter. Dark Beam will cast on the tank and deals approximately 1.5k damage. For this reason, you should have an offtank handle the adds. Unhallowed Ground is an AoE ground effect with a pentagram, and should be avoided by all raiders. The final spell to note is Destabilize Soul. This causes a debuff aura which deals 800-1k damage per second to anyone nearby. This should be definitely be kited away from the raid, and optimally to the Corrupted Souls.