Guardian of the Great Door

As an additional cost to cast this spell, tap four untapped artifacts, creatures, and/or lands you control.
Flying

Harbin, Vanguard Aviator

Flying
Whenever you attack with five or more Soldiers, creatures you control get +1/+1 and gain flying until end of turn.

Spyglass Siren

Flying
When Spyglass Siren enters, create a Map token. (It's an artifact with "{1}, {T}, Sacrifice this artifact: Target creature you control explores. Activate only as a sorcery.")

Extraction Specialist

Lifelink
When Extraction Specialist enters, return target creature card with mana value 2 or less from your graveyard to the battlefield. That creature can't attack or block for as long as you control Extraction Specialist.

Dennick, Pious Apparition

Flying
Whenever one or more creature cards are put into graveyards from anywhere, investigate. This ability triggers only once each turn. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
If Dennick, Pious Apparition would be put into a graveyard from anywhere, exile it instead.

Dennick, Pious Apprentice

Lifelink
Cards in graveyards can't be the targets of spells or abilities.
Disturb {2}{W}{U} (You may cast this card from your graveyard transformed for its disturb cost.)

Dennick, Pious Apprentice // Dennick, Pious Apparition

Benevolent Geist

Flying
Noncreature spells you control can't be countered.
If Benevolent Geist would be put into a graveyard from anywhere, exile it instead.

Malevolent Hermit

{U}, Sacrifice Malevolent Hermit: Counter target noncreature spell unless its controller pays {3}.
Disturb {2}{U} (You may cast this card from your graveyard transformed for its disturb cost.)

Malevolent Hermit // Benevolent Geist

Departed Soulkeeper

Flying
Departed Soulkeeper can block only creatures with flying.
If Departed Soulkeeper would be put into a graveyard from anywhere, exile it instead.

Devoted Grafkeeper

When Devoted Grafkeeper enters, mill two cards.
Whenever you cast a spell from your graveyard, tap target creature you don't control.
Disturb {1}{W}{U} (You may cast this card from your graveyard transformed for its disturb cost.)

Devoted Grafkeeper // Departed Soulkeeper

Runic Shot

Kicker {U} (You may pay an additional {U} as you cast this spell.)
Destroy target tapped creature. If this spell was kicked, scry 2.

Steel Seraph

Prototype {1}{W}{W} — 3/3 (You may cast this spell with different mana cost, color, and size. It keeps its abilities and types.)
Flying
At the beginning of combat on your turn, target creature you control gains your choice of flying, vigilance, or lifelink until end of turn.

Surge Engine

Defender
{U}: Surge Engine loses defender and gains "This creature can't be blocked."
{2}{U}: Surge Engine becomes blue and has base power and toughness 5/4. Activate only if Surge Engine doesn't have defender.
{4}{U}{U}: Draw three cards. Activate only if Surge Engine is blue and only once.

Serra Paragon

Flying
Once during each of your turns, you may play a land from your graveyard or cast a permanent spell with mana value 3 or less from your graveyard. If you do, it gains "When this permanent is put into a graveyard from the battlefield, exile it and you gain 2 life."

Eaten by Piranhas

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature loses all abilities and is a black Skeleton creature with base power and toughness 1/1. (It loses all other colors, card types, and creature types.)