Cavern of Souls - [Foil] Modern Masters 2017 (MM3)As Cavern of Souls enters the battlefield, choose a creature type. {T}: Add {C}. {T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

Shatterskull Pass is a pretty distant second

For example

put it into your hand

In his last seconds

then that player shuffles their hand into their library

" —Saldrath

{T}: Create a 1/1 green Snake creature token for each charge counter on Orochi Hatchery

Whenever one or more artifact creatures you control attack

where X is this creature's toughness

) Ketria Triome enters the battlefield tapped

Then look at the exiled cards and put them on top of your library in any order

And what dies grows the tarmogoyf