If I have Viktor (blue) and another unit on board, and my opponent plays Cull the Weak, can I react with Retreat on my other unit to trigger Viktor's ability and create a token to sacrifice to Cull the Weak?

Ruling: Yes, this interaction works correctly. When you react with Retreat on your other unit, Viktor's ability triggers before Cull the Weak resolves, creating a token that can be sacrificed to Cull the Weak.

Sequence:

• Opponent plays Cull the Weak
• You react with Retreat on your other unit
• The unit gets bounced back to hand
• Viktor's ability triggers before Cull the Weak resolves
• Viktor creates a token
• The token can be sacrificed to Cull the Weak