Very Rare Item: have I got the power level right?

[Ionathus]

Darkness as a reaction seems potentially OP. I'd suggest you drop that (and maybe make it DC 16). Anything thats "as a reaction" is going to be open to abuse.

Or go nuts and call it an artifact.

Eh, it has a resource cost. Maybe the reaction cast could be more expensive (4 charges?), but I don't see it breaking anything even as-is. Especially if your entire party doesn't have Devil's Sight, since you'll likely be mucking up other people's plans until you can move on your turn.