He can do it multiple times, and it's not a tap effect, nor a mana requirement
Number 53 Language English Not Legal Formats Alchemy, Brawl, Commander, Competitive Brawl, Historic, Legacy, Modern, Pauper, Pioneer, Standard, Timeless, Vintage Rulings (2/29/2020) If Enter the Dungeon is cast during a subgame, a sub-subgame begins, preferably under a new, smaller table, but do what you can
Later game you can play Silverquil while holding up an instant and use Silverquill as casualty fodder in response to removal
Artifact Color tag templates(None) Dragon's Hoard Whenever a Dragon you control enters, put a gold counter on this artifact
At the beginning of your upkeep, remove an oil counter from Archfiend of the Dross
And when we talk about issues with Reserved List cards going forward, its worth clearly differentiating between cards from this era and cards from, say, Urzas Saga