Tiamat + Dracogenesis + Cloudstone Curio

Prerequisites

If Tiamat is in the command zone, you have enough mana to pay its commander tax.

Steps

1. Cast Tiamat either from your hand or from the command zone without paying its mana cost plus commander tax, if applicable.
2. When Tiamat enters the battlefield, it and Cloudstone Curio trigger.
3. Resolve the Cloudstone Curio trigger, choosing to return nothing to your hand.
4. Resolve the Tiamat trigger, causing you to search your library for up to five Dragons and putting them into your hand.
5. Cast any Dragon from your hand without paying its mana cost.
6. When the Dragon enters the battlefield, Cloudstone Curio triggers, returning Tiamat from the battlefield to your hand.
7. Repeat.