Mono-Red Klothys, or my take on mono-red prowess.
Explanations of card choices:
Soul-Scar Mage, Monastery Swiftspear: The core of the deck is playing these 1-drops and buffing them and swinging in. Legacy is a format of efficient answers and lean threats, so usually getting them to 3/4 or 4/5 is plenty large. We play 1-drop threats specifically because we want them to trade favorably with removal and to be able to dump hands and get in quickly if necessary. Sometimes people will run Blistercoil Weird as additional copies of this effect. Never side out.
Bedlam Reveler: The best gas in the deck. If it resolves you get a 4-for-one in cards, often drawing you into another reveler as well. This helps enable the midrange plan and usually lets you see more cards than any blue deck. Vulnerable to graveyard hate, although making eight land drops isn't unheard of and usually isn't worth siding out when you think there might be graveyard hate. Generally a little too slow against fast combo.
Lightning Bolt, Chain Lightning: Flexible removal spells that can also go face for extra reach. Usually your prowess creatures represent more damage than burn spells alone and they're used to clear blockers, but can also take care of planeswalkers or finish players off in a pinch. (That last point is why I don't run Lava Spike.) Never side them out.
Faithless Looting: Secretly the best card in the deck. The normal usage of just filtering cards and fixing draws is useful, as this deck does need the right mix of creatures, lands, and spells in order to function. The negative card advantage is generally offset by being able to have the right cards at the right time, by fueling Reveler better than anything else in the deck, and also by pitching Punishing Fire and/or other copies of Looting as "free" discards and regaining actual parity. I usually wait to cast it as long as possible unless I already have something in hand I'd like to pitch. Never side this card out.
Light Up the Stage: Pretty simple, 1 mana draw-two. There are a number of awkward hits (especially Fireblast and Bedlam Reveler) but usually it's enough gas to keep you going, and of course the prowess triggers help. This also has the benefit of being a 1 mana spell that's castable through Chalice of the Void on 1 or through Sanctum Prelate. It does encourage you to point burn at face, so I tend to side out some copies in matchups with lots of creatures you want to hit.
Grove of the Burnwillows + Punishing Fire
: I think I might still be the only one who's tried this so far, but the card has been great to me. Repeatable removal against creature decks, a clock against noncreature decks, free discards to Faithless Looting for card advantage, and of course more prowess triggers. The loop is surprisingly easy to assemble with four copies of each card and twelve card draw/selection effects, and there's about a 1 in 6 chance to have them both in your opener too, which feels even better, and both cards still work by themselves in a pinch. This gives the deck near unlimited uses for its mana. Wasteland and Back to Basics are technically live against the deck now but I've never really been punished badly for it. Trim some copies of pfire against fast combo but don't cut it entirely.
Spikefield Hazard
: A new addition to the deck, a great piece of flexibility. 1 mana for 1 damage has always been fine in a prowess deck (This is replacing Lava Dart), but the option to play it as a land instead does wonders for consistency. You get to choose whether to run a 16-land deck or a 20-land deck as you're playing without boarding lands in or out. The spell half also has some neat upside against Uro, Titan of Nature's Wrath (one of my least favorite cards ever) by making it exile to its own sacrifice trigger. Don't side this out.
Fireblast: 4 damage for 0 mana is a very good rate. Kills Batterskull tokens, Thought-Knot Seers, Muxus, Goblin Grandee, and of course opponents. It's great at punishing combo players after they cast Ad Nauseam or Doomsday. Everything else about the card is awkward. You want to have access to one to close out games, but it's a dead card in the early game, drawing multiples is bad, exiling it to Light Up the Stage is awkward, the deck only playing 12 Mountains is very awkward sometimes (except when siding in Blood Moon) and you just really don't want to be sacrificing your lands most of the time with all this deck's mana sinks. I wouldn't recommend a playset unless you're running a faster, more explosive build of some kind with more Mountains and less grind engines. Side it out in almost any fair grindy matchup except maybe Goblins because of the aforementioned Muxus.
Manamorphose: Does a number of things decently. Fixes mana to make Grove of the Burnwillows and Klothys, God of Destiny less awkward, gives "free" prowess triggers, and adds spells to the graveyard for Bedlam Reveler. Side it out if you aren't bringing in Klothys and don't want to side out business spells.
Abrade: In the maindeck here because Chalice of the Void or Sanctum Prelate on 1 is such a beating for this deck. This is a flex slot, and I've been known to run Pyroblast in the maindeck instead when it has more hits. The card selection in this deck means it can usually afford to run some situational cards like this that you can move around between main and side as needed.
Roiling Vortex: Another meta call/flex slot. These days it's live against so many decks (including some of our bad combo matchups) and both hate effects are live in some way or another. Hits for the second ability include Force of Will, Daze, Omniscience, Aluren, Lotus Petal and Mishra's Bauble and Lion's Eye Diamond and the other 0 mana artifats, Hogaak, Arisen Necropolis, Snuff Out, Surgical Extraction (on our pfire for instance). Hits for the third ability include Batterskull, Oko, Thief of Crowns, Griselbrand, Scavenging Ooze, Umezawa's Jitte, Klothys, God of Destiny, Swords to Plowshares.
Blood Moon: Against some decks that rely on nonbasics, like RUG Delver or Lands, landing this is lights out. Also good against Dark Depths and Cloudpost decks, both of which are very bad matchups. Not usually worth bringing in except in those situations.
Klothys, God of Destiny: Extra grinding power for grindy matchups. This mono red deck actually has eight green sources so casting it isn't particularly difficult, and the upside of an indestructible enchantment that clocks, gains life, and ramps is great.
Grafdigger's Cage: Very wide hate card that's good enough to side in for almost any combo matchup, especially Storm, Elves, Reanimator, Dredge, Hogaak. Don't bring it in against decks that incidentally might get hit like Goblins or Maverick.
Karakas: Another wide hate card that helps in some really bad matchups, namely Reanimator, Hogaak, Show and Tell variants, and of course Dark Depths (including Lands). Usually isn't enough to swing any of those matchups but can win you games you couldn't have otherwise.
Pyroblast: Side it in against blue. Answers Oko, Thief of Crowns better than anything else. Also bring it in against storm to hit their cantrips.
Surgical Extraction: For all the really fast combo matchups where you need an answer starting turn 0. I also like to bring it in against 1 card combos like Dark Depths, Doomsday, and Show and Tell just in case I get to exile their win condition.
