A player has a high level character in a game but fails to show up for a long time. How would you go about retiring that character from the game?
This is a pretty open-ended question. As the Dungeon Master you can create all sorts of reasonable causes for a character to depart the group. It's far harder to close a campaign and retire all the characters, than to send one particular player's character on it's way. I've found that, just by using the player's own play style as a guide, you can create a quick scenario to see them off. Priest's become High Priests of their religion - no more adventuring. Paladin's become Lords, Wizards find a town that needs protecting/ has some magical font or source that needs dedicated study, Thieves form/take over a local Guild, etc. Lot's of ways to remove a character without rancor. Now, if the Dungeon Master and/or group never liked the player in the first place, it could get mean…
If a high level character is kept around the rest of the party will want to use that character as a slave to do their bidding so I try to get rid of the character as soon as possible through any sensible means necessary and casually tell the players to 'move on'.