This post contains SPOILERS for Heart of Thorns, so read at your own risk.
There are several hints, and I’d say outright statements, according to which the Mouth of Mordremoth in Dragon’s Stand is the physical representation of Mordremoth itself. Whether this means the Mouth is Mordremoth’s original form or simply a vine from its body made in its likeness to act as a feeder is debatable, though, thanks to Laranthir’s dialogue box during the battle which muddies the waters. Either way, the Mouth is still more directly connected to Mordremoth than anything we’ve ever seen before for multiple reasons:
1) The great battle outside distracting Mordremoth during the final story mission
During the Hearts and Minds mission as we infiltrate Mordremoth’s lair, we hear the dragon’s roar (which is translated as “Armies cannot stop me” if you play as sylvari). Canach comments the following: “The dragon is focused on the battle outside. We’ll never get a better chance.”
This, to me, heavily implies if not outright states that the Dragon Stand meta (or at least partly; see point 5) is taking place simultaneously with Hearts and Minds, and as it does, it means the Mouth of Mordremoth is, for all intents and purposes, Mordremoth. What other battle of great enough scope would Mordremoth be fighting outside against “armies” if not the Dragon’s Stand meta? The battle near the Heart of Thorns tree forces Mordremoth to divide its attention between dealing with our guild and the Pact’s forces, at least initially (see point 5 for more discussion on this topic).
2) Similarity between the shadowy visage in the Dream and the appearance of the Mouth
If you look closely in the background during the fight against the avatar of Mordremoth (and during the cinematic of entering the Dream in which you and Mordremoth taunt one another, e.g. “Bold words, but empty ones.”), you see a shadowy visage which looks exactly like the Mouth of Mordremoth. This further gives us reason to believe that the Mouth is Mordremoth itself; after all, why would an arrogant Elder Dragon project the likeness of a mere champion into its mind instead of showing itself, or an extension of itself while taunting the Pact Commander?
3) Lines of the Mouth of Mordremoth during the DS meta
During the fight with the Mouth of Mordremoth in Dragon’s Stand, you hear several lines, each angrier than before the closer you get to beating it. Unlike the dragon champions/minions that we’ve seen in game so far, the Mouth actually uses “me/my” instead of “My master/the Jungle Dragon/Mordremoth.”
Based on all the evidence we’ve seen in core game, living world seasons and the expansion so far, no dragon minion, no matter how arrogant or how high in the hierarchy of minions, would ever dare to place itself above the Elder Dragon it serves. All dragon minions fanatically follow and downright worship their dragon master to sickening levels, so why would the Mouth, if it were a mere champion, suddenly deviate from this pattern and want the people it’s fighting to serve it rather than its master?
Some of the notable lines which show the Mouth’s uniqueness in this regard (and thus pointing towards it being Mordremoth himself) are the following lines, taken during the DS meta:
Mouth of Mordremoth: Who dares defy my eternal will?
Mouth of Mordremoth: You are nothing. Submit to me, and serve!
Not exactly the lines an obedient and fanatical dragon champion would say, especially so close to its master’s lair, right?
4) The ending cinematic of the DS meta
The Mouth of Mordremoth, although weakened after the battle in Dragon’s Stand, is still very much alive and is retreating to the Heart of Thorns tree. All of a sudden, in the ending cinematic of that battle, it has what looks like a stroke, releases magic from its body and falls on a spike which impales its head, after which players can access Mordremoth’s Vantage POI and collect loot.
The way the explosion and the “stroke” are set up is because the Commander struck the finishing blow in Mordremoth’s mind at that moment (which resulted in a similar explosion that released magic from its body in four directions), thus crippling the dragon’s physical manifestation at a crucial moment. We know from every other dragon champion that they continue to function even after their dragon master is dead (e.g. see Risen in Arah explorable who still preach the glory of Zhaitan, and Tequatl growing in power and continuing to attack more furiously after Zhaitan’s demise as the events of Tequatl Rising from LW Season 1 were confirmed to take place after Zhaitan’s defeat). So, if the Mouth were a mere minion, it wouldn’t have had the stroke and died like that; we would’ve simply continued the fight and hacked it to pieces instead of seeing it happen in a cinematic.
The reason why the explosive release of magic looks greater in the story ending cinematic than in the actual DS meta finale could have several reasons: 1) the game engine would not be able to depict it accurately and thus shows a more limited view of it, and 2) the explosion’s scope is made ambiguous enough so both interpretations for when the battle takes place are valid due to the way it’s been written (see point 5).
5) Laranthir’s lines during the DS meta
If you talk to Laranthir during the Dragon’s Stand meta (on one of the floating islands), he expressly says some interesting stuff about the Mouth of Mordremoth, and his dialogue changes depending on if you’ve already completed the final story mission or not. The relevant snippets:
Laranthir: (before completing story) Our scouts believe the creature before us is feeding on ley-line energy for its master, but Mordremoth is an entity of mind and dream. That’s where you’ll have to go to defeat it.
Laranthir: (after completing story) Though you killed Mordremoth within its realm of mind, the physical manifestation of its hunger still remains in our world, still feeding on this hub of ley-line energy.
Note the use of the word “believe”, which indicates the scouts only assume things without knowing the full truth, and the phrase “the physical manifestation of [Mordremoth’s] hunger” when referring to the Mouth later on. One could try to argue based on these lines of dialogue that the Mouth is a mere minion, but the evidence we’ve seen above (in points 1-4) shows otherwise.
At most we can say regarding the minion “theory” is that Mordremoth grew the Mouth out of itself (hence the long, vine-like body for the Mouth), the Mouth is connected directly to Mordremoth and allows the dragon to possess it to taunt enemies when needed. But for all intents and purposes the Mouth is, as evidence seen in game suggests, the physical form of Mordremoth we fight regardless of semantics, so we did get to fight an epic battle against an Elder Dragon both physically in the open world and in the centre of its mind.
Sources for Laranthir’s dialogue:
Before completing story
After completing story