What mission in the game do you unlock the sabastion side
#Sebastian unforgivable spells
tribal cloak
dusky arch
While Sebastian's, friendship quest will start reasonable early into the game: his side quests regarding the unforgivable curses start much later into the game - at least at the half way point, and well into late game.
Continue following the main quest and Sebastian's quests as they appear.
copper halo
They all begin with "in the shadow of"
tribal cloak
Ok