The Cult Overlord can be found in the third Chapter BogHeim, at Depth level 6. You will encounter sub-bosses at Depth level 2 and 4.
This boss only has one phase, but he gets consistently more aggressive along-side the Dungeon threat level rising which means more aggressive and difficult enemies will also spawn in. I found this boss to be more a race against time to beat him before the threat level reached maximum.
I went with the second Magic Wand, the one that looks like it's covered in Flame and fire for this chapter, simply because it allows for ranged attack and a rather decent DPS. Feel free to use whatever you're comfortable with, but also note I beat this in SP so I had to roll with whatever gave me the highest DPS to beat him before he reaches full threat.
He will start by jumping around the arena; whenever he lands a small shock wave will release which can do damage, so jump over it. Once he has jumped around 5/6 times he will start slamming the ground releasing shorter, more frequent shockwaves. Dodge these and he will be stunned meaning you can wail on him with your best attacks.
Once he stops being stunned, he will summon a giant wooden object which will land somewhere about the arena. Don't let it land on you, avoid it, and then destroy it. It will send shock-waves out when he jumps which means you have to dodge multiple shock-waves. You're just making it a lot easier on yourself if you take it out.
This tactic will suffice the whole fight. Dodge his jumps and shockwaves, attack him and then destroy his wooden totem. The hardest part is defeating him before the threat level reaches the highest level.
A useful tip; at the start of the dungeon at Depth 1 there will be some altars you can light which either make the game harder/easier at the expense of points. These points don't really matter, but sometimes you can get some really good buffs like "Glass cannon" which means you deal more damage but have little health. Another good one is LifeSteal which heals you as you deal damage.