Armored trolls are trolls that have been tamed, strapped into heavy armour and trained for war. They belong to the Dawnguard, who use them as extra muscle in their crusade against the vampires of the Volkihar Clan. The idea came from Gunmar: while fighting an ordinary troll he wondered what could possibly be worse, and the armoured troll was his answer.
For the Dragonborn, these beasts are less an enemy than a purchase. Gunmar will sell you one, and it then trails you around Skyrim as a sturdy, if not very talkative, follower.
Where to find them
After the quest A New Order is finished, a small wooden pen holding three armoured trolls is built at Fort Dawnguard. If you sided with the Dawnguard, one of the trolls joins the assault on Castle Volkihar during Kindred Judgment. A troll that survives that battle stays on for good, either near the boat landing outside the castle or inside the keep.
Getting an armored troll as a follower
Some time after A New Order, Gunmar offers armoured trolls for 500 gold. He refuses the sale if you already have a non-humanoid companion, such as Meeko or Vigilance. Buying at a higher level may get you an armoured frost troll instead of the standard version.
Like other animal followers, the troll only understands three commands: wait, follow and leave. If you tell it to wait and stay away too long, it gives up on you and returns to the wild. Hitting it by accident makes it hostile, but sheathing your weapon calms it down again. Gunmar warns otherwise, yet in practice the troll gets along with any NPC who is not hostile to you. Against a wild troll it can be outmatched: a low-level armoured troll may flee and hide after being roared at.
Notes
Vampire players are not entirely shut out. With Destroying the Dawnguard active, casting a strong calm spell such as Harmony or a dual-cast Pacify inside the fort makes the Dawnguard peaceful enough to talk and trade. Gunmar will then sell a troll even while Serana is travelling with you, and Dawnguard bolts and spell tomes become available too. Curiously, killing every troll in the pen does not stop a purchase; a fresh one still walks out to meet you.
