Bloodcast (Whenever this deals damage, the next spell you cast this turn costs less for each point of damage dealt.)
CascadeRecruit [N?] (Whenever this attacks an opponent, exile cards from the top of your library until you exile a creature card [that costs less]/[that costs N or less]/[with less power]/[with N or less power]. You may put it onto the battlefield tapped and attacking that opponent. Put the exiled cards on the bottom in a random order.)
RebelRecruit N - (Whenever this attacks an opponent, you may pay . If you do, exile cards from the top of your library until you exile a creature card [that costs N or less]/[with N or less power]. You may put it onto the battlefield tapped and attacking that opponent. Put the exiled cards on the bottom in a random order.)
KaaliaRecruit N (Whenever this attacks an opponent, you may put a creature card from your hand with power/CMC N or less onto the battlefield tapped and attacking that opponent.)
Another totally off-the-wall thing, if we want to play up the Valhalla-esque aspects of Nordic flavor and/or make this more a shard of undead (which is feasible if you in turn make Neter more ruled by spiritual undead and make Valheim home to the physical ones):
Battleraise (During combat, you may cast this from your graveyard. [If you do, you may have it enter the battlefield tapped and attacking.] Battleraise only one card per turn.)
Legendary Creature - Octopus Horror
Infect
Whenever Lorthos of the Darkslick attacks, you may pay :8mana:. If you do, Lorthos of the Darkslick deals 8 damage divided as you choose among up to eight target creatures and/or planeswalkers. Tap all creatures dealt damage this way.
8/8
Next: Unifying Theory
Asinine Vengeance 1WU
Instant
Tap target creature.
Asinine Vengeance deals damage to target attacking creature equal to the number of tapped creatures.
Machine Shade 2UB
Creature - Shade
B: Machine Shade gets +1/+1 until end of turn.
When Machine Shade dies, look at the top X cards of your library, where X is its power. Put one of those cards into your hand and the rest on the bottom of your library in any order.
1/1
Creature - Elf Warrior
Infect
Whenever Tangle Prowlers attacks, it gains "Tangle Prowlers is unblockable" until end of turn if defending player controls three or more artifacts.
3/3
Next: Feudkiller's Verdict
Voracious Teachings BRGW
Sorcery
Voracious Teachings deals 1 damage to each creature target opponent controls. You gain X life and may search your library for a card with converted mana cost X or less, reveal it, and put it into your hand, where X is the total damage dealt this way. If you searched your library this way, shuffle it afterwards.
Splintershape 4UB
Creature - Thrull Shapeshifter
As Splintershape enters the battlefield, you may destroy target creature and have that creature's controller reveal his or her hand and discard a creature card. Splintershape enters the battlefield as your choice of a copy of the destroyed creature or a copy of the discarded card.
0/0
Well, we could always dispense with targeting and say "a player's library." Fateseal does it that way. I don't know, but the idea of "Drown N target player" sounds weird.
Not a fan of Pilfer as a name. I don't think the flavor's green enough, given that it's the green shard. I think, working within GUB, a workable name/flavor would be Scavenge?
Granted. If you want to ape the Skaab, you could always go with:
Different!Scavenge (As you cast this, you may exile a <thing> card from your graveyard. Reduce this spell's mana cost by that of the exiled card. Mana cost includes color.)
And then have cards that key off the scavenged cards. I think Drown would lead to a more coherent shard, though.
Hmm.
"Drown N" (needs a better name) (To drown N, put the top N cards of target player's library into his or her graveyard. You may cast one card from that graveyard this turn.)
Riffing off milling:
Mill Convoke (As you cast this, you may put any number of cards from the top of your library into your graveyard. For each card, reduce this card's cost by [or by one mana of any of that card's colors].)
Would need to be costed somewhat more expensively than convoke, but still feeds into a self-mill theme, like the Skaab and other Innistrad blue zombies.
Voidscream Nephilim GWUB
Creature - Nephilim Horror
Hexproof
Sacrifice a permanent: Return target permanent to its owner's hand.
4/4
Perhaps make it self-targeted only?
Gravescry 2 (To gravescry 2, look at the top two cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)
Primarch of the Crushing Seas 4GUU
Creature - Efreet
Flying
At the beginning of each upkeep, you may return target permanent to its owner's hand. If you do, you gain life equal to the returned permanent's converted mana cost.
5/5
Sorcery
((P/W) can be paid with either W or 2 life.)
You gain 6 life.
Planeshift Rapacious One
Brighthoof Knight 3G
Creature - Beast Knight
When Brighthoof Knight enters the battlefield, if you control a red permanent, it gets +2/+0 and gains haste until end of turn.
As Brighthoof Knight enters the battlefield, if you control a white permanent, choose a color. Brighthoof Knight has protection from that color.
4/3
Predestine makes for a more sentient, scheming Essya than we've thus far envisioned, but I think the mechanic works really well as-is, and then we just need to tweak the flavor a bit. Something to do with an instinctive descent into savagery, perhaps?
For Hala'o as the storm shard, I like it, but don't necessarily love it just yet. Alternatives?
Creature - Wizard
Flash
Infect
When Oilcaster Metamage enters the battlefield, target instant or sorcery spell gains infect until end of turn. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
The Praetors taught him of time, and death, and fire, and how to sheathe them all in corruption.
2/1
Next: Angel of Salvation