From the codex :
Finite Life
This creature cannot heal, regenerate, or gain life. If the creature gained Life before it gained this trait, it keeps that Life but cannot gain additional Life. All Nonliving objects have Finite Life.
Growth (Druid vs. Necromancer) for [mwcard=DNC06]Kralathor, The Devourer
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A creature with a Growth condition marker has grown in size. Each Growth marker provides Melee +1 and Innate Life +3. This marker has no removal cost, and cannot be removed by spells or effects which remove condition markers.
Innate Life +X (Druid vs. Necromancer)
This creature has an increase to its Life, which usually represents an increase to size or mass, and bypasses the Finite Life trait.
[mwcard=DNJ03]Etherian Lifetree[/mwcard]
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Questions (as I would answer them):
Finite life can sustract all +x life of my kralathor that have before hi put his altar of skulls?
No. What you have gained, you keep. Innate Life isn't affected anyway.
The same question about Etherian, if i have eTherian befor altar i loose all my +2 life?
Etherian Tree grants Innate Life so it bypasses Finite Life. You keep the +2.