Dawnguard. Guide to Obtaining the Armored Troll
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Remember there were rumors that you could get a pet armored troll in the game? Well, it's true, you can get it for just 500 gold.

All you have to do is choose the side of the “Dawn Guards” and prove your loyalty. After completing all the trials, Gunmar and Sorina will take you to Ismar - the leader of the “Dawn Guards.” After talking to Ismar, go to Gunmar and buy the pet armored troll from him for 500 gold.

Now you have a pet that does not count as a companion, which means you can walk around with your partner and your new friend.

Apparently, the pet armored troll is a sort of substitute or rather a reward for those who did not choose Harkon's side and his blessings. After all, the lord is a vampire and summoning gargoyles is still better than just swinging an axe, and now you've been given a cute troll with decent attack power. For example, he can easily handle the guards of the entire city. Two or three hits and the enemy NPC is already on their knees, so this is a worthy replacement for the vampire lord and gargoyles that you can obtain by following the vampire storyline.