Both consumed sacrifices to summon something, but Netherworld Whisper was special in that it leaned more toward an unknown "descent" than forming a contract with a summoned creature.
Although the descent was one-time only, its effects were dozens of times greater than those of contracted summoned creatures.
There was no realm threshold and no need to form a contract. By relying on a sacrifice to bring about large-scale destruction in a single use, it was a top-tier spell that completely surpassed ordinary summoning magic from every angle. But beyond that, it had a rather strange side effect.
Beneath this forbidden spell, Merlin had specifically noted a passage:
"Netherworld Whisper is highly likely to attract an entity from the 'True World.' The reason It is connected to this world is unknown. To be safe, avoid using Netherworld Whisper repeatedly. This may cause Its gaze to lock onto the caster and accelerate Its descent..."
This forbidden spell itself had no side effects on the caster, but it would affect the world itself?
When Lin Qiye first saw those words, he instinctively felt somewhat resistant toward this forbidden spell. After all, every kind of power came at a price, yet the price and effects of Netherworld Whisper were clearly disproportionate. If the "It" from the "True World" Merlin mentioned really descended, who knew what might happen?
But upon thinking it over carefully, if Netherworld Whisper truly were a forbidden spell that summoned an evil existence, Merlin could never have included it in the secret tome. Moreover, the note said to "avoid using Netherworld Whisper repeatedly," which meant that using it once or twice occasionally would not have much impact. Merlin had surely tried it himself, and no accidents had occurred.
After weighing it again and again, he still engraved this forbidden spell onto