Modders have been hard at work updating Skyrim mods to work with its Special Edition re-release, and you will be—or at least should be—delighted to hear that Ravelen's Sinister Seven mod has officially made the jump. Originally released in early 2015, the Sinister Seven adds seven powerful NPC assassins that hunt you down as you level up, injecting a little tension into the main story.
"Each of the seven unique assassins is at least twice the player's level and will track them down across the map," Mnikjom, who helped update the mod for Skyrim Special Edition, writes on Nexus Mods. "They're also imbued with unique abilities outside your normal Skyrim bandit and thus may result in unusual battle tactics to win the fight. Some may be more challenging to particular player archetypes, others less so. They may show up when you're sleeping, when you're exploring a cave, or just on the road. You can try to fight them or run into the wilderness, which should buy you some time, but don't expect them to give up."
Normal assassins will also show up periodically, but the killer seven are the mod's main selling point. By default, the boss assassins will show up every two levels once you reach level 12, though you can tweak the mod's values to make them more aggressive. Defeating the assassins earns you custom masks with unique enchantments, as well as two armor sets.
You can find download and installation instructions on Nexus Mods.