According to the basic concepts of Yin-Yang Five Elements theory, in the Onmyoudou, the thunder element belongs to the Wood Element.
By modifying Wood Element talismans and utilizing symbols representing the Wood attribute, one can derive thunder, unleashing powerful thunder spells famous in Onmyoudou.
In essence, thunder is a derivative of Wood, as per the Yin-Yang Five Elements theory.
In the Five Elements, the element capable of generating Wood energy is Water.
Dragons, like Hokuto, are formed from high-purity Yin-type water energy. This is the source of a dragon's affiliation with Water.
Generally, dragon energy is considered Yin-type water energy of extremely high purity.
When thunder energy is enhanced by dragon energy, it exhibits explosive power similar to rapidly expanding steam, creating a formidable force.
Rozen's Sutra Thunder Spell displayed extraordinary power, comparable to an A+ rank Anti-Army Noble Phantasm.
Under the influence of this thunderstorm, Hokuto emerged from it, gradually ascending.
However, his form was unsteady, and golden particles continuously scattered from his abdomen, indicating an unstable reaction.
Despite not being Rozen's absolute familiar, Hokuto, unlike Rozen's contracted familiars, did not have the advantage of quick recovery through Rozen's mana supply.
This limitation was unavoidable even if Rozen intentionally chose to enter an absolute contract.
"Come back, Hokuto!"
Rozen called Hokuto back, and the dragon, though reluctantly, obeyed the command, dematerializing under a shower of light particles.
At this moment, Rozen and Mash landed on the ground. As Rozen knelt, his breath became somewhat erratic.
"Senpai!"
Mash quickly supported Rozen. Though initially concerned about potential injuries, Mash realized Rozen was only experiencing breathlessness and fatigue.
This condition was familiar to Mash.
"Is it a shortage of magical energy?"
Indeed, Rozen's magical energy was showing signs of depletion.
"Even with Chaldea providing magical energy to the seven Servants, using numerous spells and rituals would have depleted magical energy long ago."
While Chaldea could support basic magical energy consumption, Rozen willingly continued to provide his magical energy to enhance the strength of his contracted Servants.
Even with Chaldea's support, maintaining magical energy supply for seven Servants, and simultaneously using spells and rituals, was a remarkable achievement.
"Now, it should be reaching its limit."
Mash expressed her concern while assisting Rozen.
What Mash didn't know was, Rozen had already experienced magical energy depletion several times since the war began.
The reason was clear.
"What kind of battles are the other Servants facing on the remaining battlefields?"
Rozen had thought about this more than once. The repeated depletion of magical energy was a consequence of the ongoing battles on the other fronts.
For example, Heracles was initially doing well and didn't require much magical reinforcement from Rozen.
However, from midway, he seemed to be engaged in a severe battle, and the regeneration count of his Noble Phantasm appeared to be decreasing. Rozen quickly supplied a significant amount of magical energy to enhance his combat capabilities.
The situation with Karna was similar.
This extraordinary hero, when fully unleashing the flames rivaling the sun, consumed magical energy at an alarming rate.
Most Masters could only provide enough magical energy for a few minutes of such intense combat.
Rozen, however, continued to supply magical energy.
Later, Karna, using some unknown trump card, absorbed a substantial portion of Rozen's magical energy. The consumption was so great that the thought alone was daunting.
As for Rama, Rozen faintly sensed that he was heavily injured, almost returning to the Throne of Heroes. Rozen promptly provided a large amount of magical energy to stabilize his condition.
Additionally, the ship Argo was facing pursuit by multiple dragons. Medea and Atalanta were exerting maximum effort, Mash had previously maintained her Noble Phantasm's release to counter three dragons, and Scáthach, being a magic-intensive Servant, required consistent magical support to handle numerous dragons.
Without providing full support, even the strongest would succumb.
Rozen had to supply magical energy to Kinu and Gyokuto, summon numerous Wyverns, and now, after using such powerful spells, he was left with a significant magical energy deficit.
Thanks to this, Rozen had initiated the secret art of the Crimson Wing Formation multiple times, consuming blood to replenish magical energy.
Without the infinite magical energy of a Holy Grail, sustaining such levels of magical energy consumption would be impossible.
Despite the relatively low consumption compared to explosively increasing magical energy tenfold, slowly activating the Crimson Wing Formation to replenish magical energy still required blood. Rozen had attached blood replenishing talismans to himself to avoid excessive blood loss, but he was still exhausted.
"For now, I'll absorb some spiritual energy from Hokuto to replenish magical energy."
Feeling the strain on his overtaxed body, Rozen abandoned the idea of activating the Crimson Wing Formation again and chose to absorb spiritual energy from the incapacitated Hokuto and secretly draw on the Valkyries' sigils.
However...
"What should I do next?"
Utilizing the Mind's Eye to observe the chaotic thunder within the field, Rozen sighed involuntarily.
Then...
"You truly surprise me. I didn't expect you to possess this level of power. I admit I underestimated you."
With these words, a sinister figure slowly emerged from the battlefield covered in thunder.
"Bzz...! Bzz...!"
Electric arcs danced around him, causing his body to emit smoke, and various parts of his body were charred.
Clearly, he had sustained significant injuries this time.
But...
"In the end, it still couldn't defeat you..."
A wry smile appeared on Rozen's face.
"Mn..."
Mash's complexion turned slightly pale.
Observing the two, the charred Cu Chulainn laughed.
A very wicked laugh.