Nex

Nex is confirmed to be the Zaros boss behind the Frozen door in the God Wars Dungeon. It will have over 15 unique combat moves and be in a multicombat area, like the rest of the God Wars dungeon. The rewards are confirmed to consist of the following:
 * Torva armour (Level 80 Defence requirement), a "life-point boosting armour," that has greater defensive stats than any other armor in the game.
 * An ammo-free bow as fast and as accurate as a Chaotic crossbow and as strong as dragonstone-tipped bolts.
 * New cosmetic robes of Zaros

While the other bosses in the God Wars Dungeon had godsword hilts to protect, there is no evidence at this time that Nex will drop a Zaros godsword hilt.

It is confirmed to include the z-buffer and particle graphics engineering styles to create one of the best boss sprites to date. It is also confirmed to have sound effects built in, to create a voice during the fight. Jagex claims that Nex will be the most challenging and rewarding boss to date, "You're going to need help from your clan or some friends to take it down, but huge riches await those who succeed."

The Zarosian army was imprisoned by Saradomin's forces in the beginning of the Third Age in an attempt to rid Gielinor of Zaros's following. Nex managed to escape sometime during the God Wars when Saradomin, Armadyl, and Bandos were fighting the Zamorakian Army to protect the Godsword. It took the combined efforts of all 4 armies of the Gods to imprison Nex and its army behind the Frozen door in the Godwars Dungeon, however, because of all the adventurers who came to the godwars dungeon looking for the Godsword, Nex has once again awakened and is now trying to escape from its imprisonment and this time it is up to the adventurer to contain it once again.

In the Behind the Scenes entry, Nex is described using the gender-neutral pronoun "it". Very few creatures in RuneScape are described this way, one exception being stalkers. It is unknown what race Nex belongs to, other than that it is not a member of any of the gendered races.

Trivia

 * Nex is Latin for "Murder" or "Death".
 * Torva is Latin for "Savage" or "Wild".
 * One of the twitter hints shows a plaque with the text: 'Vita brevis breviter in brevi finietur, Mors venit velociter quae neminem veretur, Omnia mors perimit et nulli miseretur', which is Latin for: "Life is short, it will end in a short time, Death comes quickly and respects no one, Death annihilates all and has pity on no one." This phrase is strongly connected to the phrase "memento mori", which literally means "Remember your mortality."
 * The room layout appears to be the symbol of Zaros.