It's an interesting idea... But, its just another way of doing a heal.
If you buffed a creature for 1 turn, I'm presuming, you cast this, end your turn, that creature attacks and loses the buff. It is in extremely rare cases when you have a creature with more than 20+ attack, one example I can think of is a Lava Destroyer played early in a FG fight. Even then, you would most likely Quintessence it to avoid the False God from using Twin Universe, Antimatter or Freeze/Rewind etc... So, if it is Quintessence'd, you wouldn't be able to use this card on it anyways.
If, as suggested by someone on the previous page, you were to use this as a way to secure Elemental Mastery... It is already too easy in my opinion to get mastery: Firefly Queen + Firefly spam, Boneyard + Fire Storm/Otyughs... combined with 2 or more Feral Bonds.
Heal would pretty much do the same as this, if not better. +20hp for a couple Life Quanta.
My suggestion to make this more viable (in my opinion), is to make Liquid Shadow only poison your opponents creatures but not poison your own - or alternatively, poison non-darkness creatures but buff darkness-creatures. Or to create a Permanent that lasts for 1 turn that makes all the damage your creatures do at the end of this turn, return to you as health.
eg.
Vampires Skull - Permanent for 1 Turn
Cost: 5 Darkness
Ability: All damage you do this turn is returned to you as health.
