Raknag summons colored circles of red, blue and yellow (sanguine sphere, cerulean sphere, sallow sphere) on the floor. You must remove both yourself and Raknag from the circles or something beneficial to Raknag happens.
Red circle does a steady drain of your health
Blue does a steady (and massive) heal of Raknag's
Yellow gives him a stoneskin.
Stand near the edge of a circle so that the next circle Raknag summons will overlap the one that's already up. Circles of different colors will combine and form a circle of either orange, green, or purple.
Orange circles produce a health and power drain.
Green circles slow your movement speed and casting speed by 100%.
Purple circles do direct damage and knock you back.
If a secondary colored circle (orange, green or purple) stays up for too long, it will turn into a black hole and one shot you. To dispel the circles, stand near the edge and wait for one of Raknag's primary circle spells. When a primary color circle and a secondary color circle overlap, they dispel each other.
Raknag may cast the same primary circle color 2 or more times in a row. Primary color circles of the same color can overlap, don't interact, and can remain up without becoming the one-shot black circle.