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If you have Wrath of Gods you may get hit with a really big disaster. If you do, abdicate and let the new ruler get the Cognomen.

The regular way to get it is to pillage a city during a war (12+ tiles).
Capture it, abdicate and bring 3+ workers to fix the city up.
You need to abdicate since it's very unlikely to not get 2000+ renown from killing units and capturing a city.
If you don't like the idea of abdicating there is a pillage duel event that may kill your leader.