Light and dark as in their simulacrums.
And I still stand with my theory. Tarddiad was one of the elder gods' older less experiencely crafted realms, (the lower planes, as Zaros would say), and it was the first Seren encountered after she left Freneskae. It is filled with (most likely) discarded elder god shards, like the ones on freneskae because it was one of the Elder God's earlier creations.
I submissively obey Lady Seren's every whim and relish every waking moment willingly serving her. She is my mistress and I am nothing without her. I am without thought, self-awareness, and being without her. She is my everything.