So, this is an all in creature combo deck, built around scaling off with Basking Broodscale.
4x Basking Broodscale
The main core piece of the deck, for which there is no substitute. This is combo piece A, and you need it to combo off.
4x Blade of the Bloodchief
This is piece B1. The combo with the blade works as follows - equip blade. Now, sacrifice a creature or pay 2 mana to put the first counter on Broodscale. Every time you put a counter on it, it produces a spawn. Every time you sacrifice a spawn, you place a counter. You now have an arbitrarily large Broodscale, infinite enters triggers, and infinite colourless mana. Because placing the counters is triggered by sacrificing the spawn, you can even use the infinite mana to play any C piece you want to win.
4x Rosie Cotton of South Lane
This is piece B2, your alternative Broodscale combo piece. Broodscale on board, Rosie enters and makes a food. This triggers Rosie to place a counter. You can now place as many counters on Broodscale as you want, creating as many Spawn as you want. Note that to trigger this again, you need to create another token, so usually you must either combo with Walking Ballista or play the Fleshraker or Altar before you start the Rosie Cotton chain.
Note that see B1 and B2 allows you to put power on your infinitely wide board of Spawn, making them arbitrarily large as well, and that means seeing A+B1+B2 is full combo.
1x Altar of the Brood
This is the deck's mill win condition. As it sees Spawns enter, it mills the opponent out. You run this to have a finisher in the Saga package.
1x Glaring Fleshraker
This card is actually the best card on it's own, but it can be hard to get it to work with the Rosie Cotton combo. Still, you need at least 1 to search out with Nature's Rhythm and it is often not bad to see.
2x Walking Ballista
This is the easiest way to combo off with Rosie Cotton, and does net you the win with Blade.
4x Kozilek's Command
So this is our main interactive card, but it's also a mana sink when you have infinite mana, allowing you to stack your entire deck and draw, effectively tutoring whichever win condition you need in the moment.
Any single copy of those cards make up the C package. A+B+C equals full combo.
4x Nature's Rhythm
I felt like I needed more ways to see Broodscale. I initially tried Page, Loose Leaf in this slot, but in this deck with 1 Fleshraker it felt underwhelming at best. It also searches Rosie Cotton. I somehow misread this card yesterday, so it isn't as good as I thought it was, but it is still the best card for the slot IMO. You could try Traverse the Ulvenwald but it is worse into grave hate and also I didn't feel like I could consistently get domain on time. I mean, it is easy if you see Saga and cast Kozilek's Command, but what about games where you see neither?
4x Giant's Boulder
These just make the deck more consistent while also smoothing out the mana. You will, ever so occasionally pay 7 to destroy a permanent, but that's honestly not the main reason we're running this card.
4x Urza's Saga
Grabs B or C, or toolboxy hate cards depending on the matchup. Also can grab Mox Opal to get a burst of mana and avoid losing as much mana in the long run.
4x Delighted Halfling
A spot of ramp, also makes Rosie uncounterable sometimes.
Saga package is pretty obvious and standard IMO.
4x Eldrazi Temple are in the mana base, explicitly for Cosi's Command and occasional Fleshrakers. I opted not to include Cavern of Souls, because realistically there are still so many places we can be countered or Consigned to stop us, and I'm not on Sowing Mycospawn to search it anyways.
