There is a very unintuitive interaction between Overwhelm and Barrier. While Barrier reads “Negates the next damage the unit would take”, Overwhelm ignores that when calculating Nexus damage.

Does barrier stop overwhelm?

Overwhelm does damage to nexus even if the opponent has a barrier..

Does overwhelm work defending?

Overwhelm will deal damage even if the unit dies. Some games might have it differently, but this is how it works in LoR. It doesn't go through if the overwhelm unit is defending, or if you use a spell to kill it since the spell technically resolves before the unit gets to attack/enter combat.