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It should go without saying, but this post will contain a *lot* of spoilers for those who haven't played a large amount of the game. You have been warned.
- Xau-Tak survived the last Great Revision. The only other 2 known entities to pull off this feat are the Dragonkin and the Elder Gods themselves.
- Xau-Tak 'killed' an Elder God. While Mah survived, it is heavily suggested that Xau-Tak is personally responsible for attempting to kill her, and leaving her in her lesser, deformed state.
- Xau-Tak is associated with cold, black stone. Some of his followers (cave horrors) wear masks made from this stone, and the Shard of Erebus - described as central and 'key' to the Elder God Wars, is seemingly made from this same stone.
- Xau-Tak has cured dragonkin of their curse. The curse was inflicted by the Elder Gods, and lesser gods have demonstrated a total inability to influence it. Only Kerapac, a dragonkin himself, has demonstrated any ability to fight it, but his proposed 'cure' is only temporary, and utilizes the player (who, as world guardian, bears shadow anima in them).
- Speaking of... Xau-Tak is associated with shadow anima! Shadow Anima is always described as being 'something else', or 'from somewhere else'. Zaros describes a 'supposed' sixth Elder God as being 'not of form, but 'something else' during The Light Within. He uses the exact same phrasing to describe Raksha, a colossus imbued with shadow anima, and himself enters the Monolith (into a realm of shadow anima) during the end of the Battle for the Monolith... presumably to search for this sixth Elder.
- Xau-Tak has successfully violated the Edicts of Guthix. He directly attacks the player during Pieces of Hate - which takes place before Guthix's death. The only other known violations of the Edicts were perpetrated by Seren, Guthix, and Zaros - the first two choosing to slumber, and the third being non-present at the time of the Edicts (and thus, either not beholden to them, or more likely... unaware of them at all, and broken by accident). Likewise, the Elder Gods also violated them - but unintentionally, as they'd been slumbering at the time by coincidence. If they hadn't - who knows? Were the Elders beholden to the Edicts of a lesser god? Highly unlikely...
- Xau-Tak references time when describing him/her... or perhaps more aptly... itself. During a flashback, Xau-Tak possesses a man, turns to the player, and states: "Do you really think you can save them, Player? You can't. The spiral of time leads only to the gaping maw of eternity. And this is Xau-Tak.". This would suggest not only that Xau-Tak can peer forward in time (temporal phenomena is usually associated with Elder Gods), but that Xau-Tak even represents an aspect of time - just as the Elder Gods themselves do.