Baldur's Gate Enhanced Edition

[Johnny Blade]

Yeah, that will happen sometimes and take half your max HP (I think).

I'm pretty sure it only happens when it rains and I've only ever had it hit people in metal armor. It's very rare, too, but in my current game Branwen actually got struck twice within two or three minutes, on the same map. Before that I can only remember Khalid and a main character getting hit once, in different playthroughs.