I thought I'd make a suggestion for a boss, just for fun, so here we go

Morgral'm:

Monster Type: Fiend

His lair can be rarely found in arcade areas with a large amount of fiend type levels, and unlike most bosses, he would have different forms for each tier you find him in. Each form keeps the attacks from its previous form, however I won't be writing them down under their attacks because I don't have a lot of time right now.

Tier 1 form

He would look like a large pit boss, except with a big trident in his right hand, as well as larger, more pointy horns.

Attacks

Impale
He thrusts his trident forward, dealing major damage to any knights or even enemies in its way.

Moves/Abilities

Backwards Jump
He jumps backwards. Wherever he lands, he deals a small amount of damage to unlucky knights/enemies that happen to be in his way.

Tier 2 form

He would look like a larger version of the Tier 1 form, except his skin is dark red, and he now has a kite shield in his formerly empty hand.

Attacks

Flying Trident
He throws his trident up, and it will come back down similar to how missiles come down in certain levels. He can only do minor shield striking attacks until he picks up his trident again.

Spiked Shield
While he is blocking, he can use dark magic to create shadow damage dealing spikes on his shield, which may impale knights who are close enough to attack him.

Moves/Abilities

Rage
When he reaches 50% health, he will gain increased attack, decreased defense, and a huge increase in speed.

Block
He blocks with his shield. This allows him to perform shield attacks, like Spiked Shield.

Tier 3 form

He would be much larger, have flaming eyes and large, dragon-like wings, and instead of a kite shield, he would have a tower shield.

Attacks

Trident Rain
He flies up into the sky, making him invulnerable to attacks, and throws multiple tridents at you from above. The tridents would be the same as the ones from Flying Trident.

Couldn't think of a name for this one
He flies up, and comes down with his trident facing the ground, which deals major damage if you are hit by it.

Moves/Abilities

Lawsuit
When he reaches a certain amount of health, he is legally protected from your attacks, increasing his defense by a lot.

Promotion
He summons various Devilites, and after you destroy a few of them, he promises the first one to kill you a promotion, buffing their attack and speed.