More from the chans:

EVERGREEN.

Evergreen plants can survive the winter months without losing their foliage.

They do not drop their leaves (die) to be reborn anew in spring.

(They) were going to destroy the population with an event (war, plague, famine etc.) in which humanity was destroyed and would need to be rebuilt.

But (they) would never lose control and would be able to reshape humanity and human culture in whatever image they wanted.

Evergreen is code for the people who would ride out the event intact.