Going to base this just off gameplay mechanics, not power level or aesthetic elements like art and lore.
It's kind of ironic to start with Briarhorn since it's not in my cube because of power-level concerns of it being too weak, but mechanically I think it's a really cool card. It introduces a lot of decisions about how you play it. It's modal, so you can evoke it for tempo or play it normally. You can use the pump offensively to push more damage, defensively to save your creatures, or just in general to trade up in combat. It can even target itself to come down as a 6/6 blocker, which is a great rate for its mana. I like that it can be played at instant speed, but doing so is a commitment towards trying to accomplish something, because EOTing it as a 3/3 is very underwhelming. Evoke is also a fun ability to try and exploit through blink or undying effects, but this isn't a card you can just put in a blink deck and flicker over and over to bury your opponent in cards, it gives you an effect you actually have to find a use for. Also while it's not a consideration the art is terrific.
Raw power in the colorless slot isn't very interesting from a draft or gameplay perspective, where I want players to make commitments and tradeoffs. Build arounds on the other hand are useful for increasing the permutations of possible decks in your environment, and if you can match it with multiple color combinations that's just added value. Containment Construct can be a fun tutor target with
Fauna Shaman, give some longevity to the more aggressive leaning red discard decks, help break the assymetry of the more grindy black discard decks, and make for some nonsensical combo decks with
Frantic Search and
Lion's Eye Diamond. It's very rewarding to build around, but you don't really need that many triggers either to feel like you got your worth.
I like how this is a pretty inoccuous card that can still punch way above it's weight. You can reasonably put this into a normal aggressively-leaning deck, but upon doing so it pollutes your decision making with certain incentives. Either you start dreaming about putting it on a high-power trampler and run over your opponent, or you start drafting pump effects or other equipment hoping to surprise your opponent with a burst of damage. Pick up a
Stoneforge Mystic and your plan starts to feel very feasible, giving you a small package of maybe 5 cards that are individually useful, but also come together to feel like an important part of your deck's identity. And importantly, the end product is something exciting that brings the end of the game closer, not a disgusting engine that buries you in game objects over the span of several turns.
This card represents a kind of value-generating ETB I like. You have to make actual considerations during deckbuilding to make the effect useful, and there's a hard-cap on how much you can get out of it. I also like narrow tutor effects, as they can function as artificial duplicates to the build-arounds in your cube, while still be desirable to multiple players in the draft. Their narrowness function as a type of build-around in themselves, and also usually helps keep them more competitive compared to the no-strings-attached versions. Ranger of Eos can find useful lategame cards even with the mana value restriction in
Shivan Devastator and
Stonecoil Serpent, interaction like
Giant Killer and
Concealing Curtains, give archetype support by fetching a sac outlet like
Carrion Feeder or token producer like
Gilded Goose, or be it's own little self-contained combo with the classic
Goblin Bushwacker from the old Standard days.
This card is more powerful than I would like, but it emphasizes something I find very desirable, which is thinking about the sequencing of your card plays and giving incentives to approach it in more unorthodox ways. This card double dips on it, making the controller think about how to play twice a turn as often as possible, which might make you play your removal spell on your own turn instead of waiting, while doing the inverse for the opponent. The card adds a lot of caveats to what might otherwise be the optimal play pattern of just curving out as efficiently as possible, and the looting effect stapled to it helps ensure more consistent games and provides a lot of synergy riders.
(+1/+1 counters as well, but +1/+1 counters often lean towards accumulation-based archetypes, which I don't find as desirable to support.)
It's a bit awkward to talk about Death's Shadow, as it's incredibly demanding of your environment. It's a build around with a relatively middling payoff of an efficiently statted one-drop, so you have to make it an auxiliary part of your deck's strategy, and there's not a lot of other desirable cards that rewards you for having a low life-total that you can actually put in your deck. This all begs the question of what this card is doing on my list, and well, we're talking about ideal cards, it's not my fault wotc isn't able to print more payoffs for it. If storm is the archetype that warps my cube environment the most, Death's Shadow is probably the second, and the reason I'm doing it is because it's the coolest archetype black has to offer. It's very hard to get away from black's inherent grindy way to play for the board, but Death's Shadow eschews all that and gives you one of Magic's most powerful creatures, if you're willing to live your life on a knife's edge. Thankfully, the support cards are mostly all "just good cards" I'm happy to cube regardless, like fetch-shock manabases,
Thoughtseize,
Ulcerate and even
Street Wraith as fodder for delve and discard triggers, so while I don't think there's a Death's Shadow deck, I do think you can draft in a way to make it a good card, which is satisfactory for me. If there's one archetype I'd consider helping out with a custom card it's probably this one. It provides a quintessentially black experience that can be incredibly tense and skill-testing for both you and your opponent.