Calling Ultima the single most consistently groundbreaking, pioneering, influential series of videogames in the history of the medium would be kind of an understatement (Akalabeth and Ultima I pioneering the idea of adapting tabletop RPGs to the medium of videogames in a way that established so many elements that would become intrinsic to the identity of the genre for decades, like the concept of grid-based first person dungeon crawling and tile-based overworld travel, later games establishing the mold for isometric top-down RPGs, Ultima Online being a critical tipping point in cementing the popularity of the MMORPG), but even among a series that consistently pumped groundbreaking videogame after groundbreaking videogame it's just completely impossible to overstate how massive Ultima Underworld's legacy stands over a huge chunk of the gaming medium, even if its effects manifest in subtle ways nowadays like I legitimately believe no individual videogame in the history of the medium has been as influential as it.
It's legitimately insane how many seemingly unrelated videogames can be directly traced back to Underworld in some form or another. Like, to get the obvious out of the way first, it established the DNA of the immersive sim genre, so if you like games like System Shock, Thief, Deus Ex, Bioshock, Dishonored, Prey, all of those games were either made by people who worked on Ultima Underworld (sometimes by the same studio even), or otherwise directly influenced by games that were.
But also, if you like any first person shooters influenced by Doom and other ID Software games? John Carmack was there when the demo of Underworld was first shown and decided he could create a faster texture-mapped first-person 3D engine. Which he did, at the cost of sacrificing true polygonal 3D environments in favor of the pseudo-3D that he would use for Catacombs 3D and Wolfenstein 3D, eventually resulting in the creation of Doom. RPGs like the Elder Scrolls or Gothic? It would take all day to list off how much they directly owe to Ultima Underworld. That videogame trope of finding out what happened to the occupants of a place through finding in-game logs and documents? It was established by Ultima Underworld and popularized by one of its successors.
That's without mentioning how many games directly cite it as an influence. Some already mentioned like Bioshock, Deus Ex, and Elder Scrolls, but also like. Half-life 2? Tomb Raider? Gears of War? Vampire the Masquerade: Bloodlines? All cite it as a direct influence in their game design.
Like of course a lot of these things are by virtue of being one of the earliest free-moving 3D first person videogames, when you're that early in the history of the medium you'll inevitably end up being the first to do *something* that otherwise someone else would eventually have ended up doing anyway, but no other game has a "first videogame to do [thing]" list anywhere as long and important as Ultima Underworld's.
