How do Heartless Hidetsugu and Gisela interact?

Question

Let's say that I have Gisela, Blade of Goldnight in play and my opponent controls Heartless Hidetsugu. We both have 20 life.

When Heartless Hidetsugu is tapped, each player takes 10 damage, which is half of their life. Gisela doubles the damage my opponent takes, so that 10 becomes 20. Gisela also halves the damage that I take, so I take 5 instead of 10.

Is this correct, or am I missing something? If this works, the Heartless Hidetsugu will always kill opponents with even life totals while Gisela is in play. If they have an odd life total, then they will be reduced to one life instead.

Answer 1

You didn't miss anything. Wizards preempted this with their ruling on Gisela

Gisela doubles damage dealt to opponents and permanents your opponents control from any source, including sources controlled by those opponents.

So it's pretty straightforward: double of a half is a whole. As you noted, the rounding could put it at 1 instead of 0, but you're playing red: surely you have 1 point of burn somewhere in your deck, right? :-)

There's no rules that say "Super-powerful combos aren't permitted".

So yes, your opponent would be incredibly unwise to tap it, although you could always Act of Treason it and tap it yourself!