Surtur

Originally a poor human graverobber from the south, his life changed when he found the Souleater, which, instead of absorbing his soul, as it would usually do, initiated a strange interaction with his body, the personalities locked inside it collectively taking over his mind (those of Flame and the Hero being the most powerful and thus influencing him the most) and turning him into a crazed indestructible fire-powered monstrosity. [Hypothetical: the fact that the Souleater acted in this way might have had something to do with Surtur's mysterious handpalm tattoos he has had from birth. Having grown up an orphan, he has no explanation for where they came from.]

Forms

First form: Surtur, the Beast

Originally, he's a regular looking human who transforms into a hulking pile of muscle made of obsidian whenever he touches the hilt of the Souleater.

In this form, he flies into a destructive rage and loses all sense of self. He's capable of impressive feats of destruction, being able to destroy entire buildings in seconds (with light to moderate effort in the case of larger buildings). This is the form in which he first battles the Wanderer.

Second form: Surtur the Obisidian

After a botched ritual meant to separate his original personality from those in the Souleater, he reaches a point where he has regular human proportions (though still more muscular than his original mortal body was) but retains the obsidian make up and power. While more collected than the previous beastly form, his behaviour is quite erratic, due to his original personality clashing with those contained in the Souleater.

Due to the same concentration of obsidian being compressed into a much smaller body (but also in part as a consequence of having his transformation under control), he is even more powerful than before, being capable of leveling entire cities in a single blast with little effort. This is the form in which he occasionally forms allegiances with his rival, the Wanderer.

After his fourth form he returns to this form, only this time it's substantially more powerful due to the exponential amount of obsidian being squished into a human-sized body.

Third form: Surtur, the Calamity

When under great emotional duress, Surtur the Obsidian undergoes a similar transformation to his first form, however with greatly increased size and bulk.

Other differences with his first form include that while the former was a relatively static hulking giant, in this state he keeps his acrobatic ability, which is actually increased. Moreover, while the original Beast remained transformed for longer periods and alternated between destructive and quiet moods, this transformation is much shorter and is in a constant state of destructive aggression.

Fourth form: Surtur the Gargantuan

Having travelled the Worldplane for millennia, he has absorbed so much obsidian to have reached a size to rival giant beasts like middle-aged dragons or the beast in the sphinx forest.

He looks like a giant version of his original beastly form. While his behaviour is more akin to his first form than his second, he still displays the intelligence of the second. This form's lack of verbal communication can be attributed to emotional detachment rather than diminished mental faculties.

Considered one of the Great Beasts.

Fifth form: Surtur the Obsidian II

Virtually identical to his second form in looks (regaining its mannerisms as well), it's however substantially more powerful due to the exponential amount of obsidian being squished into a human-sized shape. This form is the one he's in during his battle with Arúmil.

At this point he learns to use the Souleater for techniques such as drawing runes in the air with it for various effects, controlling it telekinetically, splitting it into various pieces, and flying on it like a skateboard.

Final form: The Surtur Colossus

In an alternate future where the Cataclysm never happened, Surtur eventually reaches a point where he has absorbed all the obsidian on the Worldplane, essentially becoming infinite in size.

He can be seen from anywhere on the Worldplane, and always fills the entire view from the horizon to near the top of the sky.

After an infinintely long apocalyptic rampage across the Worldplane, he was destroyed in one punch by the Wanderer, who at this point had been granted a body with similar properties to neo-obsidian by the gods, had absorbed all the ether present on the entire Worldplane, and was therefore capable of feats of essentially infinite power.

Seventh form: Neo Surtur

In a distant, possibly alternate future, in a faraway corner of the Worldplane, Surtur is revived by a civilization that has developed a technique to enhance obsidian's durability to the point of being literally unbreakable.

The form has dark grey skin with a shiny metallic hue. Its body is much smaller, slimmer, and more streamlined than other incarnations. It may be capable of destroying entire worlds. In addition, the form grants the ability to control the own body through telekinesis (in other words, flight). This happens fully instinctively and doesn't require any skill or training.

Eighth form: Meta Surtur

In an even further iteration of the previously mentioned timeline, Surtur is further enhanced into a material so compact that it goes counter to the fundamentals of the universe, to the point where no meaningful distinction can be made between what Surtur is and pure nonexistence.

In this form, Surtur's body cannot be interacted with, whether directly or indirectly, in any way whatsoever. It's unclear what feats of destruction it's capable of, but it may come close to being a threat to reality as a whole. This is possibly why the god of time decided to dismantle this timeline, an event which lead to the timeline the following two forms appear in.

When beheld, this form is seen as a completely flat, two dimensional humanlike shape, which is always the blackest black imaginable, and shows no influence from nearby lighting whatsoever.

Due to similar considerations concerning the Wanderer as Meta Surtur, the god of time decided to dismantle this timeline, leading to the timeline where the Cataclysm happens.

Ninth form: Infinite Surtur a.k.a. Surtur the Obsidian III

There seems to exist yet another alternate timeline where the Surtur Colossos managed to compress its infinite body into a finite human size, regaining its mind in the process.

Due to being an infinite object merely squeezed into a finite space, this third iteration of the second form is capable of producing an infinite amount of offensive energy.

In a way it has indestructibility even higher than Neo Surtur's, as the infinite amount of matter contained in its body makes it impossible to ever destroy enough of it to cause noticeable damage.

Trivia

The Surtur Colossus looks somewhat reminiscent of the statues on Easter Island.