Guild Wars 2

As a simple example, let’s consider an achievement to kill 100 monsters. If I kill 50 on one character and 50 more on another, will this add up? Or will I have to kill 100 monsters on the same character?

Achievements are for the account, not character specific, so you can use alts to work towards the total kills etc. The wiki also mentions using alts in the notes part.

https://wiki.guildwars2.com/wiki/Guide_to_Slayer_achievements

You can use as many characters as you like, it’ll all contribute to your progress.

As others have said, Achievements are account wide. Some achievements require you to do a series of things on one character, such as the World Completion achievement, but they are a vast minority.

Thanks, everyone!