What Happens to an Alliance if the leader leaves?

I found two useful references to your specific question:

Please note-If you are the only member of your Alliance and decide to leave, the Alliance will be deleted and you will not be able to return.

Source: https://support.smallgiantgames.com/hc/en-us/articles/360000386154-How-Can-I-Create-My-Own-Alliance-


Overview of Alliance Leadership Change Situations

To give an overview of different combinations of Leadership change scenarios in general:

I believe the way it works is that if the Leader leaves and there are one or more Co-Leaders already, one of them will automatically be made Leader.

The existing Leader can also transfer the leadership to another player before leaving.

If there are no Co-Leaders and the Leader leaves, I think the alliance becomes stuck without a Leader.

If the alliance has only the Leader, and they leave, the alliance is deleted. If you create an alliance that you’re the only member of and would like to leave it and return to it later, one solution is to create an alt account to remain in the alliance as the Leader.

If the Leader just stops playing, but remains in the alliance, then they’re a zombie forever; there’s no way to claim the Leadership, or kick them.

In general, if an alliance gets stuck without a Leader, the best thing to do is to make a new alliance and have everyone migrate. Here’s a guide on that:

Some additional references: