A child to whom Tar-Baphon was kind, long ago, when it was costless to be kind-- you're smart, you know, there's no reason you couldn't hang a cantrip, if you were less nervous, here, let me show you--
Tar-Baphon in prison, a decade later, and the child is a young man now, although still so young to Tar-Baphon's millennia-old eye. Tar-Baphon doesn't even recognize him, had entirely forgotten that afternoon, but he sneaks Tar-Baphon the keys and poisons his family and Tar-Baphon is free--
Tar-Baphon tracks the boy's family down and kills every single one of them, slow and lingering, so that they know what happens when you imprison Tar-Baphon, so that they know what happens when you hurt the boy who is Tar-Baphon's--
The boy, his shadow, eternally loyal, the only person it is safe to relax around, the only person to whom Tar-Baphon can show his underbelly and expect not to have it stabbed for his trouble--
And the boy is dead, and he died saving Tar-Baphon, and he's in the Abyss being tortured because he was loyal, and Tar-Baphon can give up everything he has for a diamond but next time there might not be a diamond and it is not acceptable that this could happen, Tar-Baphon will shake the foundations of the universe itself before he lets the boy get hurt. Tar-Baphon has long ago stopped being the sort of person who can think friend, much less love, but the boy hears it when Tar-Baphon shows him the cloak and tells him about the souls captured in it that can take the damage for him.
None of his imprisonment was as bad as seeing the broken body of the boy who would become the Mirrorgrave.