Because of the quest from the System, and the plea from Queen Shanaela, Rex gathered an army comprised of powerful individuals to tackle Shadow. He sent three Alpha Primes to aid the operation and make sure that it succeeded.
Initially, the plan was to take out Shadow and end its reign once and for all.
Killing the beast would put the Elves at peace of mind.
However, that task proved to be extremely difficult to complete, Shadow is too strong.
Not even the Demon Moon's help could allow them to overpower it completely.
So there was no other choice but to change the plan.
Instead of killing Shadow, it's better to lure it away from the core Elven territory instead.
Of course, Shadow could find a way to reach them once again but at the very least—for now, this immediate threat would be contained. Shadow is attacking the Elven villages relentlessly and also expanding its dominion, this has to stop.