How helpful is the summon Durnehviir shout?

Do I want to get it before Alduin and Miraak? Can it be OP if combined with the tongues trance perk to avoid cool down? What are the differences between the requiem shout and the vanilla shout? Does it last long enough to be useful against bosses? I can’t find shit about this shout on google for requiem. I love the concept of summoning a cursed dragon lore wise, so I’m praying this shout kicks some serious ass. Dead is dead Ultimate Skyrim btw which is why I’m not sure if I want to acquire this shout before fighting Alduin or not as I hear the soul cairn is pretty brutal tho I am using dawn breaker so idk