Dishonored Set I'm Working on.

I don't know anything about Dishonored, but here are my thoughts on these cards:

The Heart: ", : Target player reveals their hand." Power level is probably fine? There isn't any precedent for repeatable hand-looking.

The Outside: Blue/black hybrid absolutely cannot exile creatures, so the first ability has to go. I presume the second two abilities are referencing other cards, so I can't really comment on them.

Corvo: Reach on a creature with shadow is rarely going to come into effect, unless the set has a lot of cards with both flying and shadow. P/T should probably be turned down to 4/5. A french vanilla creature doesn't really feel like a legendary rare though.

The Heart: "Target player reveals the top card of their library. If it's a creature card, put it into their graveyard." and "Sacrifice ~: Destroy target legendary creature or planeswalker." However, blue/black hybrid can't destroy creatures or planeswalkers. Is there a reason it's hybrid instead of gold?

Corvo: What exactly does shadowmaster do? Does a creature only need one of those keywords to block Corvo? Can Corvo be blocked by things bigger than 4/4 if they have one of those keywords? Also, shadowmaster is an incredibly complex ability to be a keyword. I would recommend just writing it out, and if you're planning to put it on multiple cards, uh, don't. Fear and intimidate aren't used anymore anyway. I like the second ability, although it should be worded "Destroy target planeswalker with four or less loyalty counters on it."

I'm glad to hear it, I enjoy this too.

Since The Heart is a mythic, there's some room to increase its power level. I would make its mana cost and the first ability simply ": Target player mills a card." The last ability should be worded "Sacrifice ~: Destroy target legendary creature or planeswalker."

I think limiting Corvo's ability to block shadow creatures by power is strange - I would change him to say "~ can't be blocked by creatures with defender." and "~ can block creatures with shadow." Reach should also be capitalized.

Ok, here we go:

The Outsider (and Outsider's Mark): So Outsider's Mark is a keyword ability that doesn't do anything? I feel like that's just a design mistake. I would change everything that has Outsider's Mark to have "outsider counters" or something instead so it's easier to remember. The Outsider's ability should be changed to "Put an outsider counter on target creature."

Corvo: "~ enters the battlefield with an outsider counter on it." Also, the defender and shadow-related abilities should be on different lines.

Domino: Red can't directly destroy creatures. It should be black, or possibly white. It could also stay red if it damaged those creatures instead of just destroying them.

Outsider's Mark: It should be an Aura. You don't need to capitalize legendary, creature, or first strike. To match the other updates: "When ~ enters the battlefield, put an outsider counter on enchanted creature." Finally, it just doesn't have a mana cost? That should probably be fixed. CMC 1 is probably fine.

Void Whale: You can't have a 6/6 for four mana without a downside, or I suppose if all four of the mana are green.

Mesmerize: Ok, I think this syntax should work:

Enchantment

When ~ enters the battlefield, put a stun counter on each of up to two creatures.

Creatures with stun counters on them can't attack.

At the beginning of each opponent's end step, that player may remove a stun counter from a creature they control.

Notice I made it white. Preventing attacks is a white effect, not blue. It's also a weak enough effect that it can only cost one mana. It's basically Guard Duty but it only lasts for a few turns.

Oracular Order Sister: I think I understand what you're trying to do?

Creature - Human Cleric

~ enters the battlefield with two +1/+1 counters on it if you control another creature named Oracular Order Sister.

~ gets +1/+1 as long as it's blocking a creature with an outsider counter on it.

It might be too strong at 2 mana. Not sure.

Brigmore Witch: "~ enters the battlefield with an outsider counter on it." Does it have to be two colors?

Blink: Needs to be an Aura. Granting both reach and shadow is generally not super useful as it only helps block things that have both shadow and flying. It could just say "enchanted creature can't be blocked" and that would be fine.

Billie:

-You can just say "you" instead of "Billie Lurk, Void Walker's owner".

-The second ability could probably cost -2 instead of -4. It should be worded "Return target legendary creature card from your graveyard to your hand. Target opponent discards a card."

-The last ability should be worded "~ deals 9 damage to target player. That player sacrifices all creatures with power 2 or less."

Mesmerize:

-Doesn't need to be an Aura since it doesn't enchant anything.

Rat Plague:

-If it creates tokens equal to Plague Rats, you need to spell it out: "Create a black Rat creature token with "This creature's power and toughness are each equal to the number of creatures named Plague Rats on the battlefield." named Plague Rats."

-Enchantments don't tap. If you really want it to only work once per turn, you need to write it out.

-What's Weeper? Does it put it onto the battlefield from your library or your hand? Is it a token?