Ma Zhu's upgrade Ancient Wrath: "I fear you may not be balancing the powers of the Gods," Ma Zhu tells you, hinting at this upgrade. "Abaddon is a dangerous one".
Amenhotep's Genesis Upgrade: "Foolish Mortal! You have let Abaddon grow too powerful," He scolds you. "We must undo what has been done! It will take time. Do not let Abaddon know"
The Ancient Morgulis's description shows Abaddon speaking to the player about death: Approaching the monument, you begin to feel as if killing monsters is a natural thing in life. It is as necessary as breathing, and eating. A voice startles you from behind. "Death is good" he says. You turn around-it was Abaddon. He was following you. "Their death brings you happiness and gold. It feels good, does it not?"
The Ancient Kumawakamaru's description mentions Abaddon as well: A darkness radiates from the monument, causing many things to become difficult to see. You catch a brief glimpse of movement nearby... Was that Abaddon? Upgrading this ancient will allow you to sneak past monsters when exploring.