Chainbreaker Demon Creature - Demon Trample Soulbond At the beginning of your upkeep, sacrifice ~ or a creature paired with it. You rarely see the servant of a demon. At least not alive. 7/7
Spirit of the Tarm Soulbond Creatures paired with Spirit of the Tarm have "This creature's power is equal to the number of card types among cards in all graveyards and its toughness is equal to that number plus 1." 2/1
In a cycle alongside Spirit of the Forge, Spirit of the Confidant, Spirit of the Piledriver and Spirit of the Snap.
If you don't mind, I'll use an old card of mine I like. It would require a rewording of the ability from "pair this creature" to "pair this permanent", which is fairly minor.
Possessed Blade Artifact - Equipment Soulbond CARDNAME can only be attached to the creature it is paired with. Equipped creature gets +5/-3 and has first strike and trample. Equip It begins with the blade claiming: "You're my master. I exist to serve you." It ends with the wielder repeating the same thing to the blade.
It was revealed to me in a dream. All the cards were in my dream except for three slots which I plugged in as Vault Skirge.
FULL DISCLOSURE: The dream was post AVR so there was a copy of Slayer's Stronghold. I decided to keep it pre AVR in my post here.
EDIT: I just remembered. The Skirges might have been 2x Whipflare and 1x something else, probably War and Peace.
EDIT2: Currently 4-0 in my first four games with this. My opponents have been very angry. I fought off a Wurmcoil from Grixis for six turns before I found my second Dispatch. Jeez, I can do a lot of damage.
EDIT3: Streak broken. Lost to an unexpected Overrun. Cracking a Shrine for 10 wasn't enough.
Seraph of Judgement Creature - Angel Warrior Flying, Soulbond When Seraph of Judgement enters the battlefield, destroy all non-paired creatures. Stand beside her, or fall before her. 4/4
Soul of Souls Creature - Spirit Avatar When you play Soul of Souls, you may unpair any number of creatures you control. Soulbond. When Soul of Souls enters the battlefield, it may pair with any number of creatures you control. 1/1
Do you think it should also have "All creatures paired with Soul of Souls have all Pair abilities of creatures paired with it."? I thought of that, but it seems like too much for CMC 5. I believe this creature is viable as-is. I wasn't really buying a lot in that set, I do have a few Soulbond creatures. I'm going to "Gather" some info to see how scary it can be.
Assuming you control exactly one of each printed Soulbond creature, here is your Soul of Souls.
Soul of Souls Creature - Spirit Avatar When you play Soul of Souls, you may unpair any number of creatures you control. Soulbond. When Soul of Souls enters the battlefield, it may pair with any number of creatures you control. Flying, Vigilance, Doublestrike, Trample, Deathtouch, Lifelink, Haste, First Strike, Reach, Hexproof, Protection from Zombies Whenever this creature deals damage to an opponent, draw a card. : This creature gets +1/+0 until end of turn. : Target player puts the top two cards of his or her library into his or her graveyard. : Untap this creature. : Exile this creature, then return it to the battlefield under your control. 8/8
Please Please Please Autocard/// / / / [c]Protean Hydra[/c] => Protean Hydra / / / I can't read the cards you reference without it... / Well, ok, I CAN read the cards, but it involves opening Gatherer, looking your card up by name, hoping it's spelled right, and...
I'm just too lazy to do it, alright? Please Autocard.
Lost Revenant Creature - Spirit Shade Swampwalk, soulbond As long as Lost Revenant is paired with another creature, each of those creatures has ": This creature gets +1/+1 until end of turn." Wandering the marsh, a lost soul ever in need of companionship. 1/1
Serviceable Centaur Creature - Centaur Soulbond As long as Serviceable Centaur is paired with another creature, both creatures have horsemanship. "Give me your courage and I'll give you my legs"
Cracking a Shrine for 10 wasn't enough. 
