During your turn, every spell cast before Tendril is counted then result is x2 = drain damage : it can be any instant, sorcery, enchantment, artifact (normal or mox or buggy artifact land) casted and put in battleground...I don't think abilities are counted in (mobs, enchantment, artifact, planeswalker activation abilities) though Rule Engine does

If you want an ingame example, make a duel against an AI tendril (for eg. take the deck used for Challenge 4), and the AI will do it once (never though he could do it, he tutored many manasource and blasted me for a whooping 17 dmg...then passed all turns

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