It seems that dropping a well of radiance on top of the central plate that you use to prevent Dul Incaru's wipe mechanic causes you to deal no damage to the boss (after you pick up the orbs from the knights of course). Is this intentional?
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Not a bug. The "Wipe Mechanic" you talk about is caused by the Orbs you pick up, or rather their buff. They enable you to damage Dul Incaru, but in turn will kill you if you do not cleanse the damage buff before their timer runs out. The central plate is where you cleanse the buff. No buff means you can't damage Dul Incaru.