[FrankTrollman]

It's not random. At least, it doesn't have to be. If all five slots are filled and someone casts a new global, the check goes in order:

1. If there is another global enchantment that costs the same type of gems, it fights that one.
2. If there isn't one, then it is random.

So if someone casts Wrath of God, or the Arcane Nexus, or Strands of Arcane Power, your Stellar Brilliance goes away forever and is not missed. If noone casts one of those spells, then Stellar Brilliance may well hold on like grim death. Indeed, noone can shove it aside with The Gift of Health so long as The Mother Oak is in play, since the best GoH can do is kill the Oak...

-Frank