I'm just wondering if there's a certain level you have to reach when the toughest variation of enemies begins to spawn? I've seen the bestiary in the strat guide which details the level, skills, and inventory of all the variations, but I just wondered if there was a hard and fast rule as far as the level you need to reach to see them.

On that same note, does anyone know (or have a link to info) which enemies in the game simply level with you? I thought I read something about the Dragon Priests doing this.

You'll start seeing really tough enemies around level 45~ I believe enemies level with you until you reach level 50.

Look at the levels of the enemies:
- Bandit marauders are level 25, expect them commonly around level 30.
- Melee draugr deathlords are level 30 so expect them commonly around 35.
- Ebony archer draugr deathlords are level 40 so expect them commonly around 45.
- Arch warlocks (the highest enemy mage variants) are level 46 so around 45 from my experience.
- Vampires scale right up to 58+ with the highest variants, the new dawnguard vampires.
- Ancient dragons are level 50, but I commonly see them around 45.