Guild Wars 2: Mist Champions

Guild Wars 2: Mist Champions an all-new element for the Stronghold PvP mode that will alter the gameplay associated with the mode. In Guild Wars 2: Mist Champions players will be allowed to select highly specialized champions which can be summoned to turn the tide of battle. There will be three Mist Champions to choose from:

Nika - Obsidian Flame Assassin: Long-time Guild Wars fans will remember this hero. Nika has the handy ability of being able to stealth a whole team at once and their lord. Nika, you'll need to own Heart of Thorns to unlock her.

Turai Ossa - Savour of Elona: This battle-hardened warrior can summon a protective barrier to aid the defense of friendly players and their Lord. Turai by default once Stronghold is released.

Grymm Svaard - Sailor and Sword-for-hire: Supported by his flagship Nomad II, Grymm has the ability to call in cannon barrages which burn and knock down any enemies they hit. Grymm available to all players but they'll need to complete his reward track in PvP.

Players can replace heroes in Stronghold being able to select one before they enter the match. Each Mist Hero has a unique War Cry as well as a passive and elite skill. War Cry's are used when the Mist Hero is first summoned and provide a map wide buff. For example, Turai Ossa grants a protective barrier to all team members, Grymm Svaard calls in cannons to burn and knock down foes and Nika will pulse stealth around herself. Selection of these Mist Heroes and tactical use of them is going to be incredibly important, with teams needing to choose the right one based on opponents they're facing.
I guess that ArenaNet will continue to add Mist Heroes to Stronghold in future content updates, with all future ones requiring Heart of Thorns.