How to get regeneration serum?

There are a few ways to get it.

During the RoC events the Institute of War building will generate 20 a day for you although only up to a set limit that you have stored. 60 max in general but 300 if you buy a package.

Killing Wight King enemies during RoC will give you 20 a day for the first kill and then you can also get 1 for every wight king that your alliance kills.

Completing various challenges during the event will gain you more. Check the quests and Season Challenges.

You can also buy serum in the institute of way building with honor points.

And also buy packages that give serum through the in game shop.