The Eredar Twins saw eye to eye with Evenson on this matter. Their approach to the Fel Scar Rift was simple: lie low, use it sparingly, and unleash it during war for maximum effect.
Although they had now conquered the Fel Scar Rift and commanded nearly a hundred thousand demons, giving them some confidence, they had never thought themselves invincible or believed they could simply crush everything in their path from now on.
The Burning Legion's power was undeniably formidable, but these were only the forces of a single Fel Scar Rift. They could not represent the Legion as a whole. The Fel Scar Rift was already a place forgotten by the Legion; its troops were no more than a drop in the ocean compared to the entire Legion.
Furthermore, the rift's strongest fire support consisted only of Gernos's Fel Cannon positions. They had no way to guide massive weapons of destruction like Argus's Doom Star. They had no units like Fel Battleships, either. Though Gernos had mothballed over a thousand engines and could build them himself, he had not built any yet!
There were only a handful of powerful demons before them, barely enough to count on one hand. Charging in with just these forces would probably mean... Evenson thought he had the advantage, Evenson went in, his momentum was unstoppable, Heroes of the Storm, hero slain, and amid laughter and cheer, he typed gg.
Even if Evenson and the Twins would certainly run away and hide first when that time came, it would still mean one battle sending them right back to square one, leaving a complete mess behind. It simply was not worth it.
Therefore, the correct approach was to treat it as a secret weapon, an unexpected force. They would lurk here in secret and build