Story related, impossibile to miss if you're going to beat the game.
This is the second to last mission in the story: after you've branded all 5 Warchiefs in the 2nd map you can reach this final location.
First of all you'll have to win a battle against 5 mini-boss (same strenght of normal Warchiefs), but you have your branded guys with you, so you'll beat them in a snap.
The final battle is against The Tower. This can seems tricky at first, but actually you just have to be carefull: you cannot face the Tower directly, or he will brand you and kill you in 3/4 times (depending on how hight is your health bar).
What you have to do is to sneak behind him and brand him. The Tower is present in multiple avatars, when you brand one of them the avatar will disappear.
Use ghost vision to seek him out, then have a look to his path. The Tower usually stands and turn his head after a few steps; so keep covered behind a pillar and take your time. When he starts moving, just reach him and brand him with
+
.
Do it 3 times and you'll got it.