There's another movie, albeit an older one, that can serve as a pseudo template for at least part of a DnD movie....actually a series of movies.....Indiana Jones
When Raiders of the Lost Ark came out I was in my early teens and boy did Harrison Ford making his way through the temple look familiar to this young dungeons and dragons player
He wasn't in armor and he didn't fight monsters, really, but the dungeon exploration was fantastic
Big Trouble In Little China would also work well as a template. It's got sort of a comedic version of the same tone and it does have a couple of monsters (including a beholder knockoff). Also, Lo Pan has the general traits of a ravenloft darklord: evil, magic powers, wields great influence over an area but is trapped there, object of his desire perpetually just out of his grasp
Lo Pan: My problem is this place. This place is my tomb. I'm buried here. A young man, a king, a warrior, is entombed in this old man's crippled body. And all I need is a woman, Mr Burton, a special kind of woman with dragon-green eyes, to make me whole and young again, so that I may rule the universe from beyond the grave.
Wang: Ch'ing-ti. The God of the East.
Jack: Who, him? This guy?
Lo Pan: No, not me, Mr Burton. My demon. The god I must appease in order to regain my heart, my blood. A girl with green eyes to satisfy Ch'ing-ti, a girl brave enough to embrace the naked blade. When I find her, I will marry her.
Wang: Never!
Lo Pan: Ch'ing-ti will be happy and my curse lifted.
Jack: You can rule the universe from beyond the grave.
Lo Pan: Indeed!
Jack: Or check into a psycho ward, whichever comes first...What? I'm supposed to buy this ****? 2,000 years and he can't find a broad to fit the bill? Come on, Dave, you must be doin' somethin' seriously wrong.
Lo Pan: There have been others, to be sure. There are always others, are there not? You seem to be one who knows the difficulties between men and women, how seldom it works out. And yet we all keep trying, like fools.