Shay Shay Shay, Shake Your BOO Three

{temawimag}
Here’s a couple images of treasure chests, though they probably won’t exactly fit what you’re looking for:
generic chest
discovered loot
gold at the bottom of the sea

Phantasm: still not really a fan of this
Madness: sure

Card comments:
Immortal Mythology could be 3 or even 2 mana (you need to cast 2+ spells off of it for it to be worth even three mana, and it's hard to set up)
Surreal Nightmare is quite a bomb- a beefy body with evasion that dodges removal that removes a creature on ETB, and turning all your dead cards into removal spells is massive
Regal Siren needs an ability cost and/or stat adjustment, compare Courtly Provocateur

Wording comments:
I think the "into" should be capitalized in All into the Depths?
Mistform Phantom needs to specify "under its owner's control" (Otherworldly Journey)
Immortal Mythology's replacement effect should be "instead exile it" (Ravenous Slime)

Favorite card: All into the Depths

{MrLozano}
Ripple and names were bold themes to choose for a large set, but I feel it may have turned out poorly.
Ripple worked in Coldsnap because Coldsnap was a small set. With 60 unique commons, there’s around a 76% chance of there being 3 or more of one common opened, and around a 57% chance of four or more. With this set, there will probably be 105 commons (10 per person + the 5 land cycle), which puts the chances of opening 3 or more of one common at 40%. Note that this is just the chances of opening and doesn’t take into account someone else taking the card or you passing it for something better (and also doesn’t take into account the inherent randomness of Ripple).
There’s a couple potential solutions.
1. Make sure the Ripple/name cards are playable just as 1-ofs. example Widewood Elf, Cardmancer’s Trick
2. Increase the things that play with names at common. example Mistform’s Disciple. I’d suggest moving Temple of Names up to uncommon and one of the artifacts down to common as a start

Card comments:
I think Collective Uprising is playable without the first strike
Firstborn Warrior is an example of a card that’s p bad without 3+ copies
Before the Jump is probably not common. it also might be able to be two mana? since it can’t mess with opponent’s stuff
Jade Seer has repeatable scry, which is very good. Repeatable scry 3 is pretty absurd
I love the naming game with Mistform’s Disciple but I’m not sure it’s fine at common or at 1cmc
Reinforced Denial is p much unplayable as a five mana hard counter. You could have it start at 4 with a minor upside?
Midnight Uprising is undercosted, since it’s a “deal X damage to target player or planeswalker, where X is the power of target creature card in your graveyard plus 2” with a lot of upside. I’d start at 3cmc without the +2/+0
Bernd’s Invocation could get a P/T buff easily
Sage Beastmaster has the potential to draw a massive amount of cards, but even drawing 1 is enough. This effect is a lot cheaper and has a lot more potential than similar variants (see Temur Ascendancy, new Kiora)
Master of Names: better clone should not be two mana, I’d start at four

Wording comments:
For all of the cards with flavor text, if there’s a person speaking it they go below the rest of the text (put cursor before the dash and hit shift+enter)
Strength of the Kin has a name typo (strenght) and needs an Oxford comma. I also think it should be “each other creature you control that shares a name with it and has…”
Before the Jump is missing a period and I think needs adjusted reminder text to apply to the card being cast?
Mistform’s Disciple needs a comma after Disciple (“Disciple, choose”)
Reality Beacon needs a comma after spell and needs to choose a card name (“spell, choose a card name, then…”). The name check needs to be if it has the chosen name “If it has the chosen name, put it…”). Otherwise needs to come first in the last clause (“Otherwise, put it on the bottom…”)
Reinforced Denial is a sorcery instead of an instant
Midnight Uprising: or after end step (“beginning of the next end step or if it would leave the battlefield”). comma after turn in the trigger (“this turn, that…”). I think you can also drop “this turn” and say “it” instead of “that creature”
Vampirous Bite: commas after way (“this way, you…”)
Bernd, the Storyteller: “choose three nonland card names”
The Placeholder: comma after battlefield. “until you reveal a creature card. Cast it without paying its mana cost, and put the rest on the bottom of your library in any order.”

Favorite card: either Temple of Names or Mirror Invocation

{Gurf}
Card comments:
Spirit Possession is pretty bad, compare Cartouche of Solidarity
Spirit Warden could prob get a small stat buff
I can’t tell if Diabolic Imp is costed correctly or not, it seems somewhat weak as-is
Infernal Clergywoman still doesn’t feel super black (love the FT though)
Devilish Summoner: not really a problem but an imp that summons another imp is odd. was thinking demon but I don’t think that works with your flavor
Ritual Dagger: I don’t think this needs the life loss
Decrepit Chapel: this could have a random upside (gain 1 life?) if you want

Wording comments:
For FT with a speaker, use the Big Dash (idk) which is autocorrected from two hyphens. You can also space it with shift+enter
Fire-Slinging Imp: “dies, it deals”
Priest of Feasting: see above