This is probably really subjective and can probably be overcome with skill/OP items but...
ONLY ON FLOOR 1/2
This thing is one of the most broken first floor bosses in the history of existence and I wish it would die/be nerfed. Seriously, most anyone who has played the binding of isaac: rebirth can tell you how literally awful this boss is to face early-game if you have any plans of tryna get a deal with the devil (something that gives usually great items but has a small chance of being accessed if you take red heart damage).
Now to the boss
Fittingly named The Haunt, this boss will give you nightmares.
1st phase: Arguably the more tame phase of the two, the haunt kinda floats around the room while he sends 3 Lil' Haunts (one of the fastest and most accurate enemies from my experiences, earlygame.) At first he sends one, which is okay but not super hard, and then he sends 2 after you when you kill the 1st one which for some reason is leagues harder than just one if you have no speed/damage upgrades, which isn't an unlikely situation, considering how early this thing spawns.
2nd phase: After killing the Lil' Haunts, this happens:
This thing flies up to the wall and proceeds to become one of the most annoying pieces of crap ever, moving left and right on the top wall. The haunt in his 2nd phase has many potent attacks, that all do good damage. It's moderately fast, arriving about the same speed as Isaac's base speed, and that isn't even the fastest thing about it. I've gotten slightly better at this boss fight ever since I first fought him but i still find this thing extremely unfair for the basement unless you use azazel.
Other things i hate about The Haunt:
- One of two bosses who has access to brimstone shots on floor 1
- He can quickly reverse his direction without telegraphing in a lightning-fast charge attack whih is very, very hard to avoid.
- You HAVE to have either a ton of skill or a select few items to reliably stomp this thing; otherwise I find it unfair