If possible, assimilate the cube's essence in a violent explosion or some showy entertaining display of power. No one will know it was yours, as even the god who holds it is unsure who it belongs to... Unless he tried to peer inside. I wonder if a god can be made mad...? Its possible that whoever first discovered it had to have looked inside for at least a moment... Perhaps they weren't driven to full insanity, but its possible a few screws became loose... A few possibilities there...
All in all, I like the idea of spreading plague to Uivars, befriending the city dwellers, and creating an off-shore sentient sea-born race who will expand and do our dirty work. Hell, who knows what's underwater? There might already be sentient life deep down, or some unnamed horrors previously undiscovered. (RL deep-sea creatures are fucking frightening and oftentimes massive)
With that plan, we will attack this world on three fronts. False compassion for the city-dwellers, giving them the idea that we are a heavenly gift sent to aid them in beating back the Uivars. Second, assimilating the Uviars to gain loyal warriors with which we can harrass the city and then swoop in to destroy them, or in some way use them as bait to improve or status, and/or pressure the city-dwellers to embrace us by making them fearful of their safety. Third, create a sea-born race where no mortal man may reach us, to slowly procreate and expand without being hampered by Man-made ambitions. This can come later. For now, we can simply speed up the process of their sentience and, rather than control them, simply use a bit of power to make them more intelligent and increase their fertility... If they do not live up to our expectations, we will not have wasted all of our belief in a useless creation, AND, we can ABSORB their essence by taking them into our incorporeal body like coal to a furnace. (Good ability, and it makes sense too. Perhaps this can be learned by our Mad-God?)