mtg secret lair sonic bonus cards Spoilers — June 27 This better be the bonus

Blizzard Specter Blood on the Snow Chilling Shade Draugr Necromancer Grim Draugr Gutless Ghoul Hailstorm Valkyrie Kaldring, the Rimestaff Narfi, Betrayer King Priest of the Haunted Edge Rime Transfusion Rimebound Dead Zombie Musher

I guess Craterhoof Behemoth is a thing, too

The primary effect you want here is the card draw, and it also mills a couple cards

Verudrous Gearhulk

And with all the storm cards (and Blazing Shoal) banned, I think Ponder and Preordain would do a lot in helping blue decks make a comeback in the format

If you have experience with Kraum and have a rough idea how many cards it draws on average, you'll draw a similar amount with Mangara