Well If I can make an opinion of the matter.
I dont think the "final boss" is revealed yet, and Tanurak with Ulrissa has something to do with it.
At this point on the universe, there is a type of natural force called "Chaos" that "rules" over the world.
It seems that "chaos" repsent some type of unbalance and his natural objective is making "everything on fire"
(In my personal matter there are two forces, but we will see)
Then in all the universe there is the"material" plane where everyone lives, and the "unmaterial" plane called "The tower" where it seems is the "core" of the universe, and what happens there, affects the material plane but not backwards (at least I didnt see something like that)
The tower has at least two "places" that represents "up and down": "If you go up, you will find me" in the "holy place". If you go down it appears to be where demons live.
In this tower it seems to be there are "gears"that works on balance and unbalance of the world, two are revealed(Ulrissa hinted that there are more than two)
This two gearsare: Ginasta and Lilith.
Ginasta it seems that represent more a balancing power, where she tries to kill the "tyrants" of the material world.And right now it seems she lost her purpose of life because she know who she is and everything what happens it is part of "cycle" between and endless spyral.
Lilith on the contraryshe seems to represents unbalance, she likes to fuck everything up and she has a lot of ties with the Chaos "force",right now what she wants is posesing "the doll".
It appears to be by Ulrissa state, that if the"gears" lost their purpose, others will appear to replace them,but one "lost gear" can regain their purpose and it is more likely(talking about ginasta).
This is a personal thought but I think Lilith know her limits and tries to posses Mestan to "break the rules" and do more than what is her mission of life.
Out of that there are shards ( that we dont know who or what created them but at least it seems they pooped out of somewhere)
This shards are like "Ying-yang" thingie one represents more of a "creation purpose" (the holy power of the goddeses) and the other more like a "destruction,domination" power (the Incubus Kings) and they are the "keys" to change the world.
This shards can be united and in an "natural order" they choses specific genders, incubus shards for males, and goddeses shards for females.
This rules can be broken in the tower as we see the empress.(And probably Lilith knows that and want to do it)
And is hinted it seems that the natural order of the incubus shards are that the shards can "posses or modify" the will of their holders like "Hug the natural force of the incubus shard" but as we saw with the MC, Simon doesnt"hug" the shard and instead he wants to "dominate" and put his will on the shard.As a result of that we can see some type of "evolution"or "next stage" of the shard ( more like the true power but it is just my opinion).
We dont know if the goddeses shards works the same way, just especulate, but in my opinion it works in the same way.
In the game it is hinted that Ulrissa and Tanurak are the opposite from each other, Ulrissa seems more an observer,not interacting with the world while Tanurak wants to put the world on fire. It is like balance vs unbalance.So I speculate that they played in the "shards game" on their past, they learned too much and one got traumatized or know the responsability of this power, and the other hugged the nature of chaos.
For this and that I speculate that Ulrissa and Tanurak are the "keys"to unlock the "final boss"(some type of embodiment of the natural force that created everything, maybe a pure embodiment of chaos, or a mix of two forces that put everything in harmony,so "using" or "figting" this type of force can break the endless spiral of cycles, something like FFX shit plot, where Sin always revive no matter what so you must break the bases of this.
If tanurak has intention of getting a goddess, having a incubus shard, and wanting to"be", "take" "supplant"(by Ulrissa speculation) a gear(Lilith). maybe his goal is more than that and the role of the gear is only an step,wanthing to be the "harmony" or "force" himself.
Something curious is that in the second gathering when he introduced himself to the other Incubus Kings, for me had some vibe of "fatherly" approach to them like he was some type of powerfull Incubus King in his prime time.
And Ulrissa acting like the other goddeses being more observant and have little or no interaction with the world, I speculate she was a powerfull goddess in her prime time.(Ivala it seems she isolated herself or was kidnapped,Tertia isolated herself and didnt want to interact with the world at all,Zelica was kidnapped and isolated by some asshole,Mythin has some interaction with her cult, but she is on some type of isolated place,protecting herself from others,Grynith is the more active goddess and the youngest,but isolated as well, letting others do her will).
I still think that the "cold light"will appear before the war, or if appeared before like "The Anak" I would bet for Alonon being that.