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I'm not sure which one this is honestly, by numbers but if it's the one with the 3 bosses and then big bone... thing whatever they are called again, you have to get 1 of the little bosses health to 0, a orange/yellow aoe appears near then and someone has to pick that up (your screen will change to a teal colour when you have it) and run over to the glowing consecrated ground (there's three, one for each boss) thats the only way to kill all three then the big bone guy appears and there doesnt seem to be anything special about this phase? just like he gives the aoe ice thing so you probably dont wanna stack that and the ghosts fear (I did it on normal) so it (seems) you just fight him and surivive then he can summon the other three back in which case you repeat the process of killing them and bringing the effect to the consecrated areas.
I dont know for sure that you need each to be brought to a different one of the shrine things? it's more of an assumption.