A long time card game player, now fully committed to; maximize my MTG knowledge through playing and analysing games, and sharing this info.
Aristocrat Self Mill
Nethroi, Apex of Death is a very unique commander with the ability to return any number of creatures from your graveyard to the battlefield with total power 10 or less. To make the best possible use of the commander's effect, I decided that a self mill theme was mandatory. The lower the power of the creatures in our graveyard, the more we can reanimate with only one cast of our commander. Naturally this means we would want to build a deck that functions optimally with small creatures. I decided that the most optimal graveyard centred theme for Nethroi would be, without a doubt, aristocrat. For those not familiar with this, aristocrat themed decks are decks that aim to utilize death triggers to their advantage.
The deck's main win condition will be to drain everyone's life points through cards like Cruel Celebrant or Zulaport Cutthroat. To set up a win, we will activate our commander for it's mutate cost, to reanimate as many creatures from our graveyard as possible to then proceed by sacrificing all of these creatures with a sacrifice outlet, that can be brought back from the graveyard directly (under the form of a creature) or indirectly (using a card like Eternal Witness) with our commander's effect.
Early Ramping And Milling
Since, like with any other deck, we want to play as many spells as fast as possible, we'll need to ramp as effectively as possible whilst not hindering our game plan. To do this optimally I'd recommend using more creature based ramp (dorks) than we're used to. Birds of Paradise is really great because it has 0 power. So whenever you'll use your commander's effect, you can bring back this card for free. Other mana dorks like the popular Llanowar Elves with low power (preferably 1 or lower) are all great since all creatures can be used for our aristocrat game plan. Of course, do play Sol Ring, and maybe even a few other artifact form ramp cards, since mana dorks will get destroyed with possible board wipes and we don't want all our ramp to disappear after one.
Besides ramping, we want to fill our graveyard with as many creatures as possible as fast as we can, so that we can go for an early explosive Nethroi play. In my opinion the two best single cards to do this are Hermit Druid and Mesmeric Orb. Both can empty your deck at extremely high speeds like no other cards can, and this very early in the game, thanks to their low mana costs. However, we won't always draw in one of these two cards, so we'll need other ways of filling our graveyard. Another very good way of doing this is through a dredge engine, these aren't one-card engines like the previous mentioned two, however, since a large portion of our deck will be focussed on filling the graveyard, these won't be hard to set up. My personal favourite dredge engine is Life from the Loam. This seemingly harmless 2 mana card can cause for fast self milling in combination with lands that have a cycling ability (preferably as cheap as possible) by constantly dredging until you find what you need. All you need is to draw a card (can be during the draw step), so just filling your graveyard with other dredge cards will also get you there.
Milling For Card Advantage
Milling cards into your graveyard is a disadvantage. This is a misconception that many people, especially newer players have. However, a card in your graveyard doesn't necessarily mean that card is gone. Cards can have all types of use in the graveyard, just like we explained that a single keyword like dredge can change a card in our graveyard into an entire self milling engine. We can play some cards that can be cast from our graveyard through mechanics like flashback, retrace and possibly even escape. However I would recommend the flashback mechanic over the two others, since discarding lands for retrace can be a pretty steep cost if we don't have access to Life from the Loam, and the flashback cards like Unburial Rites are just generally better than the cards with the escape mechanic in my opinion. Also, since we try to keep a high creature count in our graveyard, their will be fewer cards in our graveyard that we'll want to exile through those cards' escape cost.
Cards like Unburial Rites, or any other card with flashback , can be considered a card in your hand. So by milling ourselves, we're netting in total cards we have access to. Some creatures can be returned from the graveyard to the battlefield just like Reassembling Skeleton, which also means we will continuously have access to sacrifice fodder or possibly blockers when needed. So now that we have covered how our deck remains capable throughout the game to continuously cast spells, we're going to take a look at cards that can keep us in the game, and even get ourselves ahead compared to our opponent's.
How does our deck actually survive long enough for us to fuel our graveyard enough to combo off for the win? Cards like the classic Grave Pact can be extremely strong in the early game since these will keep our opponent's from wanting our creatures to die and so, also keep them from swinging at us when we have possible blockers (which most of the time we will have, since attacking won't do much with our low power creatures). Besides for filling our graveyard as we discussed earlier, we'd still like to fill our hand like any other deck. Moldervine Reclamation will do a great job for this, since every time a creature of us dies, it will be replaced by another card in our hand, and even gain us 1 life, which sometimes is very relevant. Some creatures and other cards have a similar effect that draws you cards, and since just one Moldervine Reclamation won't always be in your hand, a little bit of redundancy in the deck is most definitely welcome.
Another card worth mentioning is Skullclamp. When we're in need of cards, Skullclamp can provide this for us very easily, since many of our creatures will be rather small, which means either they will die as soon as you equip Skullclamp to them, or it might keep opponent's from attacking us since we might draw too many cards from it.
Reviving Izoni, Thousand-Eyed,a sacrifice outlet and a pay-off can usually lead to a win. Let's say you can mutate your commander onto a creature you control, you have 17 creatures in your graveyard including an Izoni. You cast your commander for it's mutate cost, revive for example a Cruel Celebrant and a Poison Tip-Archer, a sacrifice outlet like Carrion Feeder or Viscera Seer and any 4 other creatures. When they enter the battlefield, Izoni will trigger. Holding priority with the trigger on the stack, you can sacrifice the 4 other creatures and Izoni herself using your sacrifice outlet (this way the Izoni trigger will create an additional 5 insects to sacrifice after). These five other creatures dying will then make all your opponent's lose 10 life through the Cruel Celebrant and Poison-Tip Archer. Then Izoni's effect will resolve, creating 15 insects, that we can then sacrifice for 30 more damage.
Saffi Eriksdotter in combination with a Karmic Guide and a sacrifice outlet can cause for infinite death triggers with any pay off in play. Saffi Eriksdotter or Karmic Guide are also really great in combination with Izoni as you can make twice as many insects by recurring the Izoni having her trigger an additional time.
Another possible option for easy infinite death trigger combos is Mikaeus, the Unhallowed. Any creature with persist that dies whilst controlling Mikaeus will trigger both persist and undying. Since you can order the triggers like you want, you can either have it come back with a -1/-1 counter on it or a +1/+1 counter on it. If you would then sacrifice the creature again, it will trigger only one ability. Every time you sacrifice that creature from that point, the creature will come back with the opposite type of counter that it had when it was previously on the battlefield.
Another way of finishing the game without having to go infinite can be done through ways of sacrificing creatures to gain mana while controlling a pay off like Cruel Celebrant. For example we either have a Phyrexian Altar on the field or a Pitiless Plunderer either on the field or in the graveyard together with another sacrifice outlet, or a combination of having both Phyrexian Altar and Pitiless Plunderer. Mutating our commander onto something on our battlefield, could return a couple of creatures from our graveyard after which we could sacrifice them all including our commander for enough mana to again, mutate our commander on Cruel Celebrant, again bringing back a bunch of creatures from our graveyard that we can sacrifice again for even more triggers.
As mentioned earlier in this article, having multiple cards in your deck that do almost/exactly the same is highly recommended like in any deck that you would build. However, since we primarily want to focus on creatures, we might end up adding a lot of creatures with "enter the battlefield" triggers. This isn't necessarily a bad thing, however, you should make sure to have other ways of removing problematic cards. For example, a torpor orb would cause all our "enter the battlefield" way of interacting with the board be nullified. Even worse would be a Hushbringer. For this reason, we have to make sure to have different ways in our deck to deal with those cards.
Another very important thing that, as a pilot of the deck, we should be trying to play around (when possible) are counterspells. Since our primary way of setting up a win is by resolving our commander and returning all the necessities from the graveyard to the battlefield, it is most welcome to have ways of dealing with these. In the abzan colour combination we have access to Silence and Veil of Summer which are very good to play around counterspells.