(They each put the top card of their library into their graveyard.) : Each player mills a card
It's a five mana 4/4, and that makes it OK in battlecruiser
Echoing Deeps Echoing Deeps is an LCI cave that taps for a colorless mana and has a neat enter the battlefield trigger

Ayara, First of Locthwain Ayara, Widow of the Realm Blightsoil Druid Briarblade Adept Compleated Huntmaster Creakwood Safewright Dawnhand Dissident Dawnhand Eulogist Deadbridge Shaman Deathknell Berserker Drider Duskwielder Elderfang Disciple Elderfang Ritualist Elvish Doomsayer Elvish Dreadlord Exquisite Huntmaster Eyeblight Assassin Eyeblight Cullers Eyeblight's Ending Gilt-Leaf Winnower Gloom Ripper Gnarlroot Trapper Haunt of the Dead Marshes Hired Hexblade Hunter of Eyeblights Infestation Sage Iron-Shield Elf Koma's Faithful Lys Alana Scarblade Maralen of the Mornsong Melancholic Poet Miara, Thorn of the Glade Moonglove Extractor Moonglove Winnower Nadier's Nightblade Nadier, Agent of the Duskenel Nightshade Harvester Plague Witch Prowess of the Fair Ruthless Winnower Scarblade Elite Scarblade Scout Scarred Vinebreeder Shadowheart, Dark Justiciar Skemfar Avenger Skemfar Shadowsage Thornbow Archer Thornmantle Striker Twilight Diviner Unscrupulous Agent Viconia, Drow Apostate

Just like with each of the booster packs you can pick from, some of the most powerful cards in the game call the Mewtwo packs their home
I am not saying this is bad, as it stops the game from becoming stagnant, but it also leads to the ban hammer coming down when cards go too far