This is the boss pack from Into The Light, a 3-phase boss immune to projectiles that will use light mirrors and different attacks to eliminate you.

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In the first two phases, the Herald Of Light will primarily attack you at close range, teleporting to do so more easily. In the second phase, it will also make use of its mirrors and beams of light.
Its third phase will reveal its true nature, mainly employing beams of light and mirrors to finish you off. It will summon a simple but tough enemy that, if allowed to accumulate, could cause you many problems.
This boss has attacks tailored for multiplayer to enhance the group attack experience