Time Spiral [Urza's Saga] Dragons of Tarkir Promos Tawnos finally cracked the puzzle:

Tawnos finally cracked the puzzle: the strange structures did not house Mishra's malevolent creations but were themselves among his creations

their bones now freeze

Set: Kaladesh Inventions Type: Artifact Rarity: Mythic Cost: {3} Whenever a nonartifact permanent enters the battlefield under your control

which can be quite unsettling for their prey

" —The Underworld Cookbook

Time Spiral [Urza's Saga] Dragons of Tarkir Promos Tawnos finally cracked the puzzle:Set: Urza's Saga Type: Sorcery Rarity: Rare Cost: {4}{U}{U} Exile Time Spiral. Each player shuffles their hand and graveyard into their library, then draws seven cards. You untap up to six lands.