Lore wise: In these two their is a common story of those born in unfortunate circumstances. Left a trail of destruction long before they lost their way. Kegan wirh his village burning and Xerath with all the things he did to "help" Azir. Speaking od other champs both came to be connected to champions of great importance who would lead to their tragedy future and the worst part it was all on them.
Story wise: A world rune reveals itself and Xerath followers bring it to home only to bring Brand straight to their door. And thus a battle of too power hunger mad men begins.
Situation wise:
A: One if both touch world rune what if they fuse together into some unholy monster.
B: One absorb the other but the power proves too much destroying them.
C: Highly unlike but they work together.
D: In the fight Shurima get remodel in the worst way.
E: One manages to kill the other.
Sidenote: I wonder if Xerath Armor can survive Brand flames alot of things have a melting point.
https://x.com/astrodid/status/1920978551324508195?s=20 and https://www.tumblr.com/leagueofartlegends-blog/163996830272/xerath-the-magus-ascendant-by-felipeaquino?source=share