I have a boss concept: Kraken
Visually: There is a normal-sized bipedal-form Tsihu with special colors to indicate how special he is. His forearms disappear into
those purple portals. He floats in mid-air surrounded by a shield.
Surrounding him are 5 giant portals(one above/behind him, 2 on each side/behind him). Out of the top portal is a large Kaiju's head, and the remaining portals connect to the Kaiju's tentacles/claws. The Kaiju arms are razor sharp(I'm split between making the "beast" part of Kraken a Kaiju or crazier version of the Tsihu's beast mode).
Gameplay: This varies, depending on whether this boss would be open world(unpredictable number of players) or instanced(5 player). The boss is essentially invincible. The Tsihu and the portals hover in place, but the tentacles extend infinitely from the portals. Each tentacle will choose one player randomly to "chase". The tentacles extend at sprinting speed and running into any section of a tentacle(razor sharp, remember?) will result in instant death. The boss essentially becomes a cooperative version of Tron. If the tentacle captures the target player, it will kill the player and seek out a new target. If a player starts running out of the range of the boss fight, the Kraken head will snipe the player with noxious acid, slowing them down and leaving them vulnerable to attack by the tentacles. If there are a lot of players, more mini-tentacles emerge from the ground to chase them
How to fight Kraken: Players who aren't being chased by the main tentacles are primarily occupied by collecting scrap parts to build bombs and distract the Kraken away from the main 4 players being chased. Parts will spawn all over the battlefield, and the untargeted players can shoot at the Kraken to gain aggro(for the acid sniping) while bomb parts must be carried to specific locations. Once a bomb gets built, 1 of the 4 targeted players can run through the bomb to destroy the tentacle. Once all four tentacles are destroyed, the Kraken completely enters the world and the Tsihu vanishes(for later player-driven events). The Kraken spews fire, snipes with acid, etc. and acts as a bullet sponge until killed.
Possible Addition features: Perhaps to make the open-world fight more vertical, the spawning of the Kraken causes large rocks and boulders to hover above the ground as normal terrain that players can jump and run on.