What have you got up your sleeve for He Who Mills, First of his Name, Phenax, God of Deception?
Alright, let's talk about this solitary journey that is Mill, courageous soul for whomst 120 life points weren't enough.
The most underrated part about a Mill deck is often the graveyard hate. A Mill deck requires much more graveyard hate than any other, else they'll find themselves only fueling their opponents' graveyard recursion tools and synergies. While a Planar Void or Leyline of the Void can fully lock down opposing graveyards, I tend to prefer softer options, options that allow ME to use my opponents' graveyards, while denying them access to it.
First, topical application of graveyard hate. You'll want it to be instant-speed so that it can fulfill its role, and to extract some value yourself from it if you can. They're not bad cards on their own (ok, Ghost Vacuum isn't the greatest at doing anything but graveyard hate), but in your deck they will keep opponents in check while profiting even more from milling them.
Sometimes graveyards get a bit too full for topical solutions to be able to contain their shenanigans. No problem, we have some wider options, and still you'll get value from them. In most decks, one would be worried to play these because they'd risk making the Abyssal harvesters and Scarab Gods sad, but in a Phenax deck, you'll be able to refuel those opponents decks in no time and for free to get more value out of them. These hit wide enough they can afford to be sorcery-speed, though Instant-Speed is still nice.
This one I pulled apart because it's actually not that good at graveyard hate. Being timing-locked to your attack step makes this irrelevant when you need it as graveyard hate. However, it's a very strong card in general, allowing you to grab any spell finisher for cheap once milled. It's a 1/3 on its own, and the effect lingers past creature-killing sweepers. The buff to toughness is incidental, but I'm sure Phenax appreciates it all the same.
I'm personally a big believer in untappers in blue, particularly ones that can target any permanent. They double as ramp by untapping lands, and if you go that route you might want to double check your manabase: they love bouncelands, lotus field, temple of the false god, anything that doubles their ramp output. In Phenax, they also each count as having the highest toughness of any creature on your battlefield, since instead of tapping for Phenax to mill, they can untap your creature with the largest butt instead. And of course, this track of mind leads to...
These are strong in Phenax, even if Phenax is your largest creature, if you enchant him with one of these, you get a repeatable mill 7 for a single blue mana. However, these are cards that WILL take over a deck and combo off if they're in the same postal code as any of the aforementioned untappers. It takes something as small as a bounceland and suddenly you have infinite mana.
Now, combos are fine, it's just one of these situations where untappers and these are individually strong and useful with Phenax, but they reach critical mass and create a chain reaction when in the same deck. If that's something you want, all power to you, I have a ton of fun with a deck that's dedicated to this kind of nonsense, I just want to make sure you're aware of this before it surprises you and a table of friends in a game. (If you do want to go the untapping combo direction I will also direct your eyes to Illusionist's Bracers. And Eater of the Dead for Phenax specifically though I assume you're aware of that one if you play Phenax.)
