ToME: the Tales of Maj'Eyal

How do you regain it?

Arcane blades and the thief ones... shadow something, they can't regain it naturally, but there are manasurge runes around which gives you a fixed amount which, if you're in a safe place can spam until back to full, or alternatively once a fight (except if you flee for cooldowns then more often).

If you can find one or two items with +0.xx mana/turn, those can boost the effectiveness of manasurge as well (since you'll get 6-12x the rate for the 10 turns or so, about doubling the effect of the rune if you have about 0.60/turn).

Shadowblades, at least, actually start with a manasurge rune. So do Archmages. I'm not sure about Arcane Blades as I don't play those.

So, the answer to the original post is most likely: use the Manasurge Rune.

Yeah, Arcane Blades start with them. And both Arcane Blades and Shadowblades can get a feed talent that gives them mana regeneration.

However it's possible the original poster has gotten some mana from an escort quest talent. In this case a manasurge rune isn't really worth using, and you're best relying on the mana restoration you get from levelling up.

Or an item that gives +x mana regeneration, which you can just equip and rest whenever you want it back.