Considered 3 life instead of 2.

Why combat phase. That's an added complexity due to nonstandard timing and it doesn't really seem worth it. Is there a life loss triggered mechanic in this set that makes this reasonable?

I needed a phase and combat phase leaned towards a good name.

Maybe something like

> At the end of each opponent's combat phase, if that player attacked with a creature this turn, they lose 2 life.

... and then possibly decrease the mana cost a little or add life gain to compensate for the fact that it's possible it won't always trigger?

Actually, that would be quite similar to Blood Reckoning / Hissing Miasma.

Suggested rarity is uncommon.

I was designing for a common slot and am not interested in uncommon designs. 2 life could be a bit strong, so maybe I should just convert Curse of the Pierced Heart to a black card and drop the curse subtype.

That sounds fine.

Also, if you make it

> At the beginning of your upkeep, target player loses 1 life.

then it shouldn't use the Curse type. Curses are deciduous btw so it wouldn't be a huge issue either way. Unless I'm mistaken, Shadows over Innistrad has exactly one curse and Amonkhet only has two.

While I don't mind curses, I don't think there's any reason such an effect needs a nerf for multiplayer.