Can Cull of the Weak be cast when you have no units on board, and can you then create a unit with another effect (like Viktor's token) to avoid sacrificing an existing unit?

Ruling: Yes, Cull of the Weak can be cast when you have no units on board. You can then use another effect to create a unit token after casting, and you would not have to sacrifice a unit since you do as much as you can.

Sequence:

• Cast Cull of the Weak with no units on board
• Use another effect (like Viktor) to create a token
• You do not sacrifice a unit because you had none when the effect resolved

Nuances:

• Cull of the Weak does not target; you choose a unit to kill
• If you don't have any units, you do as much as you can
• Cruel Patron requires killing a unit as an additional cost (not targeting), so you must have a unit to play it
• Cards like Harnessed Dragon and Hidden Blade do target, with different requirements for valid targets