Because if he didn't, the Vanguard simply lacked the raw ethereal energy to sustain the meat grinder for much longer.
He maintained the status quo for a few precious seconds, buying himself the time needed to think.
At the very least, he was still perfectly capable of intercepting and canceling the catastrophic beams the artifact fired into the sky. He 'shielded' his allies that way, providing Liora the uninterrupted airspace she needed to continue permanently incinerating a fraction of the enemy's energy.
And down on the flanks, Liu had seemingly weaponized his Wendigo's special absorption capability, discovering a viable method to cull the horde without the corruption instantly consuming his sanity.
The containment fronts were holding and they were also buying him time to think.
He couldn't afford to stall for long... But just a bit was enough.
♢♢♢♢
A moment later, the chaotic variables clicked into place. Ren reached a terrifying solution and committed to the gamble.
