Is Rune the same gameplay and mechanics as Blade of Darkness? I already have Blade of Darkness, and judging from the videos I've seen it looks like they are very similar. So I'm just trying to decide whether or not I should put this on my list.

Rune has in contrast to Severance a simple combat system. It's mostly buttonsmashing.

I've only played Blade of Darkness but if I remember correctly the reviews I had read at the time, Rune has simpler combat and the puzzles are mostly 'pull that lever/push that button' (Again simpler stuff).

Rune has less variety of blade attacks than Blade of Darkness, less type of ennemies and the levels are a bit repetitive (caverns, dungeons, castles, ice fields...).
I personally prefer BoD, which has better graphisms, 4 different characters (including a dwarf, so it's more 3,5 characters) with their own attacks and special weapons. Besides, you need a good timing to attack to avoid to lose your stamina, whereas you are never exhausted in Rune !
Puzzles in BoD are more complicated than in Rune (some hidden areas are really... hidden).

To conclude, BoD is more strategic than Rune which is a basic hack & slash !

I was going to post something about these games but this really nails it. Well said.

I will add that IMO they are both fun as hell and for their age look pretty good.

Allow me to throw in Die by the Sword to the competition

Both games are great, so you can't go wrong with any of them, though I have to say Blade of Darkness wins it for me in terms of gameplay and replayability, with 4 characters with each their own fightingstyle and weapons.
Rune has more varied leveldesign and more casual gameplay which has the really cool beserkmode, whereas in BoD even the weakest enemies can slice you to pieces if you're not careful and master the controls.