Clavileno, First of the Blessed
Hopefully, you'll have something much better to do on your turn, but if you're in a desperate spot, you might be able to use the chump blocker and the bonus card it provides to find a way out
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Yes, as a five-mana enchantment, this is a bit costly, and it would be ideal if it could get into the game much earlier, but given how out of control the counter generation can get, don't overlook that this could be used more for its haste aspect than the counters
The Ninth Doctor's ability triggers during the untap step and goes onto the stack at the beginning of the next step (usually the upkeep)
Felwar Stone is often better than a Signet in multiplayer games, and since you'll occasionally want mana of different colors, it's a nice mana rock to have