The Manga's has Gray unleashing Rhongomyniad's power for the first time when confronting The Angel of the Castle of Seperation: She basically uses it to NUKE THE ENTIRE CASTLE.
Anime Adaptation
A minor one compared to everything occurring later, but the closing minutes of Episode 1 is a good highlight of Daisuke Namikawa's voice work: Waver, whose voice for the most part has been wavering between his younger and mature voice, begins to firmly settle into the smooth baritone he will use when he chooses to accept the title of Lord.
So far, the series itself tends to be very low-powered, with most cases/conflicts addressed with pretty much mundane means. However, Episode 5 gives us a visual treat with the Marburry Workshop battle, which involves:
Kairi Sisigou once again bringing his necromantic weapons to bear against a swarm of Black Dogs, with no danger to himself whatsoever;
Wills Codlington, the arc's client, being no slouch either—engaging the Black Dogs with a pair of enchanted knives, and them never getting the drop on him;
Waver/El-Melloi II, despite being a self-professed Non-Action Guy, actually manages to provide support firepower with what seems to be something akin to the Gandr; and
The biggest eye candy of the episode: Gray unleashing Rhongomyniad for the first time. The weapon itself, when ready for deployment, looks like a hand-held laser emanating from Gray's right hand—which disintegrates the primary Black Dog foolish enough to meet it at point-blank range, and then wipes out the entire Black Dog swarm in one hit.
Overlapping with the Funny page, Episode 6 has Luvia going One-Man Army against the store's guardians with the power of pro wrestling.
Episode 8 is Mundane Made Awesome as Waver uses his hair to deflect a Servant's lightning attack! The fact that he was able to think on his feet this way (despite being in the middle of a minor Heroic BSoD caused by "Hephaestion") is quite impressive in and of itself.
Episode 9 shows us a rematch with Gray and "Hephaestion" inside a a haunted forest (called the Child of Einnashenote which, for the uninitiated into the Nasuverse, is a Genius Loci with powers ranking among the Dead Apostles of Tsukihime). That neither of them would let each other go even while the place is going unstable/haywire demonstrates their massive nerves.
Episode 12 has Gray undoing five of the thirteen Power Limiter seals that the Knights of the Round Table put on Rhongomyniad. As Add calls out each seal as it is undone, Artoria's voice is overlaid with his.
Lancelot: This is a battle that will harm no Elemental.
Bedivere: This is a battle against an enemy mightier than oneself.
Kay: This is a battle to live.
Gaheris: This is a battle that does not stand against humanity.
Agravain: This is a battle for truth.
While the anime just had vague silhouettes floating around Gray, the novel's illustration shows the five knights whose seals were released standing around Gray.◊
Gotta give props to Doctor Heartless as well; to make sure his escape succeeds, he summons a 'Child of Einnashe' using his magecraft.
What does Olga Marie do during the fight? She (along with others) uses her family magecraft to summon a Death from Above spell to attack the 'Child of Einnashe'.
Not to be outdone, the crew of the Rail Zeppelin converts the engine into a Wave-Motion Gun to use against the heart of that demonic forest.
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