So yeah. Fan idea. No pic cause Doomspeaker is lazy.
The boss is called "Revelian". He's a "Random" boss, like the Fallen and the Headless Horseman. He will only spawn on later floors because, let's face it, he's pretty damned hard.
He has a bunch of attacks.
One of his attacks consists of him charging at you. He'll signal this by letting out a scream, like Famine/War's charge cry. If he gets close, he slices you with his sword, which deals 3 hearts of damage. This is his most dangerous attack, but he rarely uses it. If you're fast, you'll avoid it, don't worry.
His second attack is, essentially, a combined version of both of Scolex's attacks. He'll run to the corner of the room, swell up, and launch a massive wave of blood bullets. 3 blood bombs are in the mix. They will not damage him if they do hit him. You can actually avoid the attack by staying close, as the bullets will fly over your head. Don't get too close, however, or he'll slice you with his sword.
He also summons 4 "Dark Matters", which act like Red Maws. They float towards you quickly, and if you kill them, they split into 3 homing shots which are aimed at you.
He can vomit a large line of creep which covers half of the room. He only uses this once, and the creep disintigrates faster then other creep. However, he can use this anywhere, which can leave you with only a small portion of the room to dodge his attacks.
Another attack has him running to the center of the room. He swells up, like his "blood wave" attack, but he starts shooting blood bullets at you rapidly, like Isaac/???'s counter attack. He also sometimes launches a blood bomb. He'll stop firing after 50 blood shots and 3 blood bombs.
And lastly, he can fire a laser beam, which looks like Shoop Da Whoop, if you line up with him in any of the cardinal directions. He grunts before he does this, so it's easy to avoid. The attack only lasts a second, so it's hard to trap him in a pattern like Monstro II.
He's got a good amount of health, and when he dies, he'll drop the "Dark Matter" power up. This gives you an all-stat increase, but it's higher then regular stat ups. It also gives you two heart containers. It changes your appearance- You have purple skin, and a single eye. This does not affect firing.