where exactly is this boss at? been looking for awhile and haven't found him yet

Is this just a reskinned magma wyrm? Any new attacks?

He have 25 650 Health (I killed him with Scarlet Rot)

he should give access to theodorix's magma at teh dragon cathedral

Located at the bottom of the frozen waterfall in Consecrated Snowfield (north of Albinauric Rise)
Had an attack that seemingly one shot me at 1.8k hp. Otherwise he's pretty straightforward, the biggest annoyance of magma wyrms are the small areas in which you must fight, but you have tons of space here.

If you can lure the giant land octopus nearby they will fight each other.

Albinauric Rise is actually in Haligtree not Mountaintops of the Giants.

I had quite a laugh with this one. When he was low health his health bar disappeared and I was confused. He was still there but had a regular mob health bar. I noticed he was fighting something. The land octopus finished him for me and then died in the lava. xD

You dont even need to fight him. Just aggro the octopuses on the way (there should be 3) and they will kill him or will at least bring him to %25-30 hp. Also they stagger him a lot.