I want to pick up some old school titles in the sale, anything similar to TES Arena / Dark souls? I already picked up one called rune, and Ultima Underworld 1+2. Does anyone know of any other really good titles I could pick up like these?

I don't really know what you have but what about:
Arx Fatalis,
Gothic 1&2
Morrowind
The Bard's Tale Trilogy
Lands of Lore
Forgotten Realms Collections including Eyes of the Beholder and many more.
Daggerfall
Might and Magic series including World of Xeen and 6-8
Wizardy 6-8
Realms of Arkania Series
Jagged Alliance
Outwards, perhaps not truly great but entertaining for what it is.
Underrail+ Expedition
Avernum remakes
Avadon 1-3
Age of Decadence
Divine Divinity
Betrayal at Krondor
Temple of Elemental Evil
Arcanum of Steamworks and Magick Obscura
Vampire the Masquerade Bloodlines
Albion

They are all games i greatly enjoy and replay to this day.

Edit: will continue to add more as i remember.

Thanks a lot for your reply. Most of those I already have sadly, not sure if there is anything else available.

If you consider Dark Souls old-school, try out Arx Fatalis. Or the Geneforge series.

Dark souls is an RPG is it? More like repetitive strain injury simulator.