Assumption and Truth
Solely speaking, Seth had not even thought of this until he heard Zera's voice.
It just so happened that according to her, Tombstones weren't rare.
But one that was semi-sentient to defend itself from Intruders wasn't common.
It started when the Tombstone exploded.
It explosion carried in mind to ward off Intruders, and that was set up by the trap master for sure.
Plus there were many more traps set up.
But after the Tombstone exploded, it gave off isolated mana.
Isolated mana was, in simple terms, mana that hadn't been in contact with the natural flow for so long that it mutated.
It was difficult for Seth to grab the key understanding but he did get the point.
Mana that isn't mana.
Now, that type of mana can be gotten in so many ways, but it is inefficient, meaning it has outlived its use to be in that form.
Meaning whatever it was being used for had ended.