Harbinger Ganon

Harbinger Ganon is the main antagonist of. It is an alternative form of Calamity Ganon that was created as a result of Terrako traveling back in time. Throughout most of the game it takes the form of Terrako's past self corrupted by Malice, but later takes a more bestial form similar to that if not the same as the Blight Ganons, and to a lesser extent Calamity Ganon's incomplete form in the original timeline.

Biography
From the Malice from the future of the great Calamity, Calamity Ganon had send parts of himself through the Time Warp portal created by Terrako to prevent any changes to its awakening. By possessing the Past's Terrako, it became the Harbinger of Ganon and was soon allied with Astor. Using what power it had, it using its malice to possess a Guardian Stalker to attack Zelda on her way to the Ancient Tech Lab and reveal Calamity Ganon's awakening in the future to Astor and the Yiga Clan.

It had lead an army of monsters to attack Rito Village which was defended by the soon-to-become Champion Revali and his Rito brethren, thus creating a paranoia among the Ritos to attack Link when they were to see Terrako with him.

Harbinger Ganon revealed itself to Zelda and her allies floating beside Astor before they entered the Malice-filled Hyrule Castle, opening up its body to draw out its Guardian weapons and claws to show intimation. It waited inside a gatehouse of the castle, hanging upside-down in a pile of Malice on the ceiling, engaging Zelda and her forces as soon as they arrived.

After being defeated along with Astor, just before Astor could call out a command, Harbinger Ganon consumed him for his power, using to transform itself into it's true form - Calamity Ganon.