Ultima
After completing 
The Rising Tide and defeating Ultima, you can examine the 
Arete Stone in the Hideaway to obtain the final, “most powerful” Eikon, Ultima.
This Eikon is another odd one to use. There’s no doubt that its abilities are powerful, but they can’t really be chained together well with other abilities, and they take over Clive’s entire ability set when you use them.
Ascension
Eikonic Feat
Channel the power of Ultima and be freed of all earthly bonds, transforming basic actions into powerful divine equivalents.
Voice of God
Eikonic Ability
Rise and call forth from the heavens a ray of pure aether to scorch the land. Can be used in mid-air.
Ability-Related Accessories
The Will of the Creator (Voice of God)
Proselytize
Eikonic Ability
Send a river of divine energy cascading before you, dealing damage to all enemies in its path. Can be used in mid-air.
Ability-Related Accessories
Dominion
Eikonic Ability
After emitting an aetherial shockwave that spreads in all directions, give rise to eight magicked blades that rush up to pierce the heavens.
Ability-Related Accessories
Ultimate Demise
Eikonic Ability
Draw in all smaller enemies, then overwhelm them with a singular eruption of magicked force. Can be used in mid-air.