Ikaruga

Just took down the newest Trial of the Crusader boss last night. Most enjoyable boss fight yet. Kinda easy, (only 2 attempts needed) but very cool mechanics. The twin Val'kyr. One is light, the other is dark. There are 4 charging portals in the arena, 2 light, 2 dark. You charge yourself with one or the other. When charged, you abosorb all damage of the same color, and deal damage to the opposite.
Throughout the fight, black and white orbs appear around the arena and seem to fly around all over the place. Getting hit by the same color starts building up a stacked buff on you. Opposing color seems to do mild damage to you. The stacked buff, however, kicks all kinds of ass. If it stacks to 100, you gain empowered darkness or empowered light. It makes you stupidly powerful. Building up this charge is something to watch during the fight. I wouldn't call it crucial, but it's very fun.
Also, the two bosses share a single health pool, and occasionally fire off a special ability. One might do a massive AOE, at which point you need to switch to the same color as the AOE before it goes off to avoid death. Another might cast a shell on herself and begin casting a big heal. Everyone has to switch to the opposing color and break the shell in order to interrupt the heal.
It's really a lot of fun, and reminded me of the old Gamecube/Dreamcast/XBLA game Ikaruga.
Video of the fight will probably be available next week.