Empyrion: Galactic Survival. We had a whole subforum for it for a while. Gas had a server he was hosting, it was a whole big thing. They've moved up to Alpha 10.5; and though they claim to have made massive improvements to multiplayer, I mostly didn't find any.
I couldn't get into Undertale for a variety of reasons, the fandom surrounding was a significant factor plus my own inability to finish roughly 80% of the games I start, but it's soundtrack is fantastic and remains on my playlist in it's entirety to this day.
Blizzard and Activision are one and the same, don't let them try to trick you otherwise. Perhaps once they ran separate from each other, but that day is no more. Hell, the full name of the company is Activision-Blizzard