Dwarf Fortress.

And, I mean pre-steam release.

The ASCII graphics, controls and everything about it were not appealing to me. I started playing it because of the Boatmurdered story and some friends trying to recreate that. We started playing and rotating fortresses on a random Friday night.

And once things started to click? I put way too much time into it. I'd eventually mod it and make it easier to play but I put many many hours into it.

I wouldeventually put the game down after that initial burst and then years later the steam release came, making it much more enjoyable and well... A couple thousand hours later I still keep going back to it.