Nice that Evil Islands (
review) and Spirit Engine 2 (
review - really, it's free, grab it! - less so for 1 (
review)) were already named... Shudder at Dink Smallwood (
review) being named though (may be free, but that's still too high a price for that one!). Wouldn't call Gothic (1 and 2) or Divine Divinity (
review) as obscure though, but sure, if you haven't played them, go ahead. Also wouldn't call Two Worlds (but the first one (
review), definitely not second (
review)!) obscure, but nice sense of exploration in that (again, referring just to the first!). Had something to hold against Driftmoon (
review) myself but ah well...
Of course, question is what counts as obscure or underrated for you. Going alphabetically through fantasy RPGs I played, Arcanum shouldn't fall under that category, Arx Fatalis probably would, not sure of The Bard's Tale (2004), Betrayal at Krondor might be if you haven't been playing games in the 90s, not sure I'd name Ember (
review) but not bad at least, same goes for Eschalon (just played 1, which is free), the Quest for Glorys (mainly 2 and 3 from my point of view) also shouldn't count as obscure or underrated but just in case, same for Might and Magic 6 (others may also add WoX (4+5) and/or 7), some sure seem to have enjoyed Nox, since someone mentioned Gothic at least the first Risen (
review) isn't bad either, some may also mention Silver, and VtM Bloodlines is well known (even more so now) but Redemption may fit for a mention...
Excluding a few due to non-fantasy.
My review links just in case you want another more detailed opinion.
Edit: Add another "know and have it but didn't play" for Serpent in the Staglands here.
Edit 2: Hm, was just going through games I finished (or clearly abandoned in one case), not also temporarily (I hope, even if it may be years) set aside or currently playing, but if someone mentioned Divine Divinity, can also mention Divinity 2 (just leave Beyond aside), not sure if King of Dragon Pass may fall under RPG, and then there's Lords of Xulima and maybe even Regions of Ruin.