WingedHao on the Valkyrie Anatomia Reddit has graciously provided a set of basic translations for the game's UI. You can view them here. Anybody can contribute to this wiki via direct edits or comments!
Enemies are weak to Holy and Ice. The event character is the new Supreme Odin, who will gain more XP, deal more damage, and add bonus event currency drops.
Event Missions found here.
This is a tower-style event where each floor has you fighting increasingly powerful bosses. Each boss beaten gives a unique reward. Some of the significant rewards are weapon limit breakers, rainbow books for limit breaking characters, gold and silver gacha tickets, and wood and iron rings for leveling artifacts. Recommended strength levels range from 1,500 up depending on floor, and usually increase by 1,000 power per floor.
Event discussions can be found on the Valkyrie Anatomia subreddit.
Advent bosses can be challenged three times a day to earn Character Potential Items.