How many things were wrong?
I like this movie quite a bit, it gets a lot of things right.
I'm sorry it also got so many things wrong, it could have been a classic for the ages on the level of terminator, robocop, back to the future etc. but it's slightly below those because of some sloppy mistakes, as if they didn't spend the proper care/time in preproduction.
I have a list here, add what else you think needed a better treatment:
1- casting: leading lady is a subpar annoying hag for mortals, immagine for an immortal.
2- story logic: is there a time limit for the "there can be only one"? If not, why even bother? If yes, why the friendship between some immortals?
3- plot hole: the sword could not have been made in ancient time. That's why Brenda follows Russel. It's never explained what the deal was (time travel?) nor how it can be connected with immortality.
4- immortal "magic powers": I always thought it's cool enough to be immortal. But in the movie they add a lot of other magic without ever explaining the logic of it.
Why does he feel the stag's heart beat on the beach? What does that do? How is that connected with immortality? What is this "quickening"? What's with the chaos every time they kill another immortal? What does that do? How do they find/feel one another?
5- minor bs: they should all be covered in scars and miss a lot of fingers, if not limbs/eyes etc. Life in the past was not office work, but manual labor. Plus, they fight a lot because they are immortal, if not taking parts in other mortal wars. Where's the damage from that piling up after centuries?