The Reaper Queen is a very ugly, bulbous creature, with four yellow eyes and thorns protruding from her body. Unlike the Elder Shaman, God King, Hee-Haw the Wicked, and Ainyl from the other Endless Challenges, she does not taunt you with words, as she is silent (possibly incapable of speech). However, she, like the others, has an arsenal of rather annoying abilities to hinder your defenses.
The Reaper Queen's most frequent attack is spewing a glob of poison at your soldiers, draining their health very quickly. Undead units are completely unaffected, and are regardless sometimes targeted by her poison. Though this damage is non-lethal (and will stop when your soldiers are at 1 HP), they will die almost instantly when fighting her minions if already poisoned, so micromanaging them to dodge her poison is a must in this level in order to keep your ground.
In addition, the Reaper Queen sometimes gives birth to a fully-grown Reaper that crawls from the temple entrance to the middle exit. At the start, it will not be a problem, as your soldiers and towers can deal with it easily, but as the rounds progress, she will do this more often, which, combined with the massive Savage hordes, can prove to be a nasty nuisance that can cripple your defenses. This Reaper doesn't give any Gold when killed.
Finally, the Reaper Queen can let out a piercing cry that gives a 6x BOOST TO ATTACK POWER of any Parasytes or Reapers currently in the battlefield, causing later waves of them to deal damage in the hundreds. They also gain small boost to movement speed and are distinguished by a spiky red aura. Even the Knights Templar can fall to them in mere seconds.