Upset
The lower-ranked entity killed the higher-ranked one. The record and the ladder disagree.
The ladder holds, and this is the only pairing anywhere on the site where the killer's own entry concedes the point before this page can ask it. Third place is an argument about consequence: about what it costs everyone else when the world loses a component it was built on. Fifth is what the ranking gives an assassin whose whole significance is borrowed from the target. Two separate entries below her make that concession in different words, which is how you can tell it is the page's position rather than a line about one character. `stated` besides — a sourcebook recounting a death, not a scene enacting one.
Cyric killed Mystra in Forgotten Realms Campaign Guide (2008).stated
Mystra's death wrecked the bonds that held the Weave together, and brought about the Spellplague.
It was the Prince of Lies who murdered Mystra and caused the Spellplague.
He is the reason the entry two places above him sits where it does. Mystra is third here for what her dying costs the Realms, and Cyric is who killed her the second time: the Weave came apart, the Spellplague followed, and both the setting and the edition were rebuilt around the wreckage. It is the largest consequence any single act on this page has produced, and it was a murder rather than a war.
Maintains the Weave, the structure through which all arcane magic in the Realms functions.
Killed during the Time of Troubles, with catastrophic effects on magic.
Killing her breaks the world, and the Realms have run that experiment twice. She is the goddess of magic in a setting where magic is a physical system she personally maintains, so her death during the Time of Troubles and again before the Spellplague did not merely remove a deity — it corrupted spellcasting for everyone alive. Ranked on consequence, which is the only entry here where the consequence is a rules change.
One death in one work. It is the only place the record puts these two in the same room, and it is evidence about that meeting and nothing wider — not a general matchup, not a rematch, and not a ranking of its own. The ladder above it is argued from everything either of them has been shown doing; this page is argued from one citation. Where the two disagree, the disagreement is the point, and it stays visible rather than being resolved by whichever of them happens to be on this page.
Both entries carry their full receipts on the Dungeons & Dragons ranking — 4 citations between them.