Since Havengul Lich lets you cast creatures from any graveyard and Heartless Summoning lets you cast creatures for 2 less; I assume that Heartless Summoning effects the casting costs of creatures in all graveyards.
Although Shadow_Seraph is correct that we won't know for sure until the FAQ is released I'm 99,9% sure that this interaction will work as normal. I.e. you'll get your 2 mana reduction when casting the creature spell from the graveyard.
DCI Level 2 Judge
Precisely why the shape and cosmology of the disc system should be so is not clear, but goes some way to explain why, on the disc, the Gods are not so much worshipped as blamed. - Terry Pratchett, The Colour Of Magic
Without being certain, it probably would. Heartless Summoning reduces the casting cost of creatures you cast, regardless of which zone you cast them from.
I hope it is true because of the things you could do with Perilous Myr, Heartless Summoning and this lich which makes it so you pay 1 mana and shock something. this nice little combo can be started as early as turn 3 also.
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