Ymir

Ymir is a Frost Giant among Giants. He's the first Frost Giant making him the most powerful and ruler of the Frost Giants. He rules the land of Niflheim in Asgard where he and his brethren reside.  He is so old that he predates any of the gods with Surtur being the sole being in the Norse cosmos older then him.

Hyborian Age

Formerly, the tales of Conan were part of Marvel history.  In these tales Ymir was worshiped as a god of war and storms by the peoples of the north before the current civilization of man rose to power.    Due to legal reasons, it is unknown if these stories are still considered canon.

Marvel History

Ages ago Odin and his brothers slew Ymir and according to Norse myth created the world from his body. 

Despite being killed Ymir has returned numerous times to plague Asgard.  Each time he has been defeated with the result being either imprisonment or death , but he constantly returns.  Several times he has been summoned to Earth by the Sons of Satannish and battled various superhero times such as the X-Men or Avengers before being returned to his native dimension.   He is a sworn enemy of Odin due to their past history.  He is also an enemy of the fire giant Surtur due to being opposing elements.  The last known battle with him left him imprisioned in the Sea of Eternal Twilight.

He did not participate in the final Raganrok and is presumed to have died with the rest of the Norse cosmos.  Considering how difficult both him and Surtur have been to kill he is expected to return.

Powers and Abilities

Despite being a frost giant, Ymir has been at times referred to as an Elemental like Surtur.  This makes him more akin to a force of nature than a god or giant allowing him to return more easily from death.

As such, Ymir is one of the most powerful beings known.  He has  all the abilities of other frost giants, but to a much greater degree.  His size, strength, and resistance to injury are far greater then any other frost giant surpassing that of even the thunder god Thor.  He can generate intense cold from his body.  He can freeze nearly anything he touches.  As an elemental, Ymir is so far immortal and immune to the effects of time.  Should his body be shattered he can mentally reform it and even if he is completely destroyed he can regenerate his body.  

In battle Ymir prefers brute force and to use a giant club made of ice.  This abilities combined with his immortality make him one of the few beings to be capable of actually challenging Odin and Surtur in direct battle with a chance at winning.

Weakness

Like all frost giants, Ymir is vulnerable to intense heat, but his own ability to generate intense cold negates all but the strongest sources of heat allowing him to survive in environments where other giants would die.

Despite his vast age, unlike Odin or Surtur Ymir has never taken the time to develop his mind or demonstrated any capability for strategy.  This makes most of his attacks a straight foward affair relying on brute force and his thinking bent on outright destruction with little else.