The basics of Materia were easy to understand. They held the wisdom of the ancients. It took a few "mana points" to cast a spell, a measure of how much background mako energy a person could draw into themselves. Larger spells cost more mana.

It was also generally understood that insufficient mana resulted in no spell cast. But even experienced users would fail to keep track or guess wrong about how much mana remained.

None of this meant too much for Cloud right now. The sheer numbers of monsters spilling out of the depths of this cavern taken a toll already, and another huge horde was gathering up even as Aerith was healing up what wounds she could and Barret carried a dazed Red XIII

"I think we're going to have to pull back for now. I'll slow the monsters down behind us though," Cloud said, preparing a red materia.

"Alright, Cloud, we'll get out, just don't bring the whole cave down," Tifa said, realising his plan.

The others started pulling away, giving Cloud plenty of space without much worry over collateral damage as he began calling forth one of the most powerful of all summons. Bahamut.

It was meant to cost 100 MP. Cloud felt like he had just enough left.

He was wrong, by 5 MP.

"Your reserves are too low, Summoner, nonetheless... I can help you, for a price," a voice spoke into Cloud's mind as he realised his mistake.

The horde was gathering, and rushing forward. They would certainly overwhelm him... and then the others would be hit from behind as they retreated.

"I accept your offer!" He said.

"Very well..."

Cloud felt the last of his mana fade... but rather than the rush of the summon appearing, he felt something else. A surge of energy throughout his own body, but clearly not his own mana refilling.

He realised he should probably have asked what the price had been, he realised, as he felt that energy grow... and felt himself growing too.

His skin began to turn black and scaly, except down his front where the scales were more yellow, and he felt things growing at his back and the base of his spine. His body grew in size rapidly, the cloth and leather of his uniform ripping as they lost the battle against his expanding form, wings and the tail ripped out even as the growth shredded the rest of the cloth blocking them, and Cloud felt his neck and lower face both elongating.

His fingers grew longer and developed long claws even as his feet ripped free from the remnants of his boots, toes similarly elongated and razor-tipped, mako-glowing blue shifting to fiery yellow.

Where Cloud had been, now stood Bahamut... but with spiky yellow hair. He blinked his now yellow eyes in surprise.

"Perhaps you should confirm what the price actually is before accepting, next time," Bahamut said inside Cloud's head.

"...Probably," he sighed, and looked at the horde of monsters.

They had all paused for a moment in surprise, but seemed about to gather their wits.

Cloud opened his mouth and with a mega-flare, didn't let them do so....