Edward sighed, stepping into the mountain with his young brother, Alphonse, following close after him. "Brother, what are we doing here?" Alphonse questioned, his metal armor clinking loudly and echoing across the walls of the cave loudly, "We're here because I've heard rumors from the nearby village that something.. inhuman lives in this mountain. Nobody told me where in the mountain, though." He frowned a bit, his golden eyes flickering around the area as they walked further into the mountain, the cave getting darker and darker with each step. "B-Brother, I don't like this place.. It's really, really scary!" Alphonse whimpered, catching up to his brother and staying close until he heard Edward yelling, and it was fading, fast! "Brother!" He called, taking one step forward and falling down a hole, hitting the ground with a loud clank! He sat up slowly and saw Ed lying on the ground a few feet away, unconscious. He bolted to his feet and raced over, picking his brother up and looking around, "Hello!? Is anyone here?" he called out, looking around frantically and spotting a single, yellow flower facing downwards.
The armor-clad boy looked a bit startled when it looked up, a large smile on it's face as it spoke in a loud, cheery voice, "Howdy! I'm Flowey, Flowey the Flower!" Flowey chimed, slowly swaying from side to side. Alphonse slowly approached it, "Umm, hello. I'm Alphonse.. Alphonse Elric." He tilted his head as Flowey spoke, "Wowie! You're tall, mister!" He chimed, looking up at Alphonse, "Who's that in your arms?" It leaned forward curiously and saw Edward in Alphonse's arms, noting his breathing was quite shallow, "He's my big brother, Edward.. We fell down here and I found him unconscio-" His sentence was interrupted by someone yelling, "Get away from that creature!" The voice called, it was female. Alphonse looked over to see a humanoid goat creature, staring at Flowey with a bit of fear in her eyes, "Come here, now!" Alphonse then realized she was talking to him. He stood and walked over.
The creature seemed relieved that he was approaching, her eyes widening at the sight of Edward in his arms, "Oh dear!" she gasped softly, "What is wrong with him? Is he dead?" Alphonse shook his head, "No. He's alive.. He's just unconscious. Do you know anywhere where me and my big brother could stay?" she nodded softly and smiled, "You can stay with me, my children. Come along." Alphonse casted a glance to Flowey, seeing the flower was gone and quickly following after the goat woman. "Hey, miss, I never got your name. What is it?" she looked over and smiled more, "I'm Toriel." Alphonse nodded, "It's nice to meet you Toriel! I'm Alphonse, and the person I'm carrying his my big brother, Edward."

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FullMetal Alchemist and UnderTale Crossover
RandomHey! Syd here. I'm going to be working on this now! It is indeed mature (Only because Edward cusses).