It is a well-known fact that for a demon to be brought across the Earth-Hell barrier, the sacrifice of a living creature must be made. So well-known, in fact, that it is the first spell that any burgeoning mage learns. Without that spurt of vital power, the most a demon can do from Hell is helplessly watch events unfold on Earth.
For example, Sai the demon watched helplessly as his Earth-born daughter was dragged onto a sacrificial altar.
The worst part was that even though he could hear her screams, he had no idea what she was saying. She'd been raised speaking her mother's native tongue, something Sai could never wrap his head around. Still, despite not understanding his daughter's language, there was one word which was the same across every language Sai had encountered.
"Mama!" Sai's daughter cried, "Mama!"
Sai flexed claws that could shred solid steel impotently as he watched the scene unfold. A zzyyanzzroth demon like him had little in the way of magic, but he had enough of an education to open a scrying pool to Earth. In the middle of the hellish, heat-blasted landscape around Sai, he could see a dank, dark cavern. Grim, hooded figures etched runes in a ring around Sai's struggling, caged daughter. A bald woman with a ceremonial axe waited next to his daughter, ready to execute her for the purposes of bringing him into their world.
Most demons would do anything for the chance to escape Hell. At this moment, Sai would have consigned himself to an eternity of torment if it would save his little girl.
A cold, detached part of his mind noted that the runes and the ceremony and the fancy axe were all entirely unnecessary. All it really took to cast a sacrificial summoning was a demon to target and an unlucky sacrifice.
He looked down at his claws.
And then a grim determination flooded him.
He didn't know his daughter's real name, of course. He'd been banished back to Hell long before she'd been born. But his daughter had a name to him.
"My daughter," Sai whispered, pleaded, begged, "My daughter, please. My daughter, be safe. My daughter, be free. My daughter—"
The cultist raised her axe to strike.
"MY DAUGHTER, I SUMMON THEE!" Rage and grief and pain and fury crystallizing into a single, quivering moment, Sai flexed his claws and slashed through his own chest, shredding his heart like an axe through an innocent little girl's neck.
A flare of magic—old magic, primal magic—left him as he gave up his life for the spell. A simple summoning spell, to take a demon across the Earth-Hell boundary. With a pop of displaced air, his cowering half-demon daughter materialized in front of him; seconds later, he heard a clang as the surprised cultist's axe bit empty air.
The burnt orange sky was fading from Sai's vision, and he knew he was about to die. The cost of a summoning spell was, of course, a life—and his was the only he had to spare. Slowly, his trembling daughter looked up, at the empty, heat-blasted plains of Hell. She looked at Sai, bleeding out onto the floor.
Her eyes widened with primal recognition, and she stood. "Papa?"
Sai smiled. "Hey... sweetie."
She clapped her hands to her mouth. "Papa!" She ran over to his side, slipped in the puddles of his blood, and fell.
Quick as lightning, Sai caught her with one dying hand. "I... gotcha, girl. I gotcha."
Sai's daughter's lower lip trembled as she pushed herself to her knees. She tried to speak, but only managed to choke out a sob.
Sai smiled sadly, and he felt his eyes close. It took a while for a zzyyanzzroth to die, but die they did, just like everything else.
"'ts okay, daughter." Sai whispered, "Wouldn't... understand you... anyways..."
And with that, Sai fell still.