Walk around in the basement until you get transported to some large arena with floating platforms and lava below. You will see a big angel called He Who Dwells, and it's up to you to defeat it! The battle takes part in three phases, and your best weapons to use are the Cinder Missiles and Phosphor Beams. There will be sources for both powers throughout the fight on choice platforms, as well as beams that propel you upwards like air vents.
The boss has two main attacks through the entire fight, one of which includes firing a laser similar to the angels from before. The other attack is a large blast that will explode when it hits you or anything else; this will increase to multiple ones in the second and third phase. Once you have caused enough damage, you will be back in the basement, so walk around until you get transported again.
For the first phase, just move from platform to platform and shoot either of the two main shots to damage him. It is the second phase and beyond that things get tricky. Once you cause enough damage, the boss will be invulnerable and the platforms will start to fall into the lava (which does hurt you), so you need to find the new stable platforms.
Once you get to the third phase, there will be angels that will create a barrier around the boss, so you need to shoot them to make them disappear (one shot should be enough). After the battle, you get the choice to either corrupt Eugene (Evil Karma) or redeem Eugene (Good Karma). When you've made your choice, the mission is over.
