黒曜石の火心
クリーチャー — エレメンタル
：猛火カウンターが置かれていない土地１つを対象とし、それの上に猛火カウンターを１個置く。 その土地の上に猛火カウンターが置かれているかぎり、それは「あなたのアップキープの開始時に、この土地はあなたに１点のダメージを与える。」を持つ。 （この土地は、黒曜石の火心が戦場を離れた後も燃え続ける。）
イラスト：
Zendikar#140神話レア日本語適正
裁定
As the reminder text indicates, whether the targeted land has the triggered ability that’s been granted to it depends only on whether it has a blaze counter on it, not on whether Obsidian Fireheart is still on the battlefield.
The ability gained by the land triggers at the beginning of the upkeep of the land’s controller, not the beginning of the upkeep of Obsidian Fireheart’s controller. The player who controls the land at the time the ability triggers is the one who’s dealt damage.
A land with a blaze counter on it is an illegal target for this ability. You may activate the ability targeting a land, then activate the ability in response to itself targeting the same land. Although the first ability to resolve will put a blaze counter on that land, the second ability to resolve won’t resolve.
If all blaze counters on a land are moved to a different land, the triggered ability doesn’t follow them. The first land no longer has the ability because it no longer has a blaze counter on it. The second land doesn’t have the ability because Obsidian Fireheart didn’t target it.
If a land ends up with more than one blaze counter on it (thanks to Doubling Season or Gilder Bairn, for example), the ability still only causes it to deal 1 damage to its controller each turn.