And a play a plains with a Emeria Shepherd on the field and I do it too the Hangarback Walker (or ofcoarse any other card that can bring creatures back to the battlefield.)
Well, yes, it did enter the battlefield, but state-based actions kill it long before you get a chance to activate the Ruins, much less resolve the ability. You can't put a +1/+1 counter on it that way.
Sadly, you can't since SBAs are checked before you can activate that ability. If instead you control, let' say Chief of the Foundry, then Hangarback Walker will enter the battlefield and live with a +1/+1 on it. Do note that this isn't a counter, but you can add counters later using its ability.
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But the enter the battlefield rule. I need to check if this will work.
Let's say the Hangarback Walker was in the graveyard
And a play a plains with a Emeria Shepherd on the field and I do it too the Hangarback Walker (or ofcoarse any other card that can bring creatures back to the battlefield.)
I know Hangarback Walker would die due to 0/0 but would I be able to tap Ruins of Oran-Rief to give it the +1/+1
In other words would graveyard to battlefield considered entering?
And can that trigger the enter the battlefield ability?
Obviously. A lot of other ways to do it so Gideon, Ally of Zendikar +1/+1 token is one way to get it before the death (I think.)
Assuming you have the emblem beforehand, yes, that would save the Walker.