Athreos, God of Passage (EDH / Commander)

First of all, whenever you have multiple Athreos triggers at one time, lump all of them upon a single person. It is much more difficult for one player to pay eighteen life than for three players to pay six life. That said, politics is one of the most important aspects of a deck like this, so be judicious. For example, I wouldn't advise targeting someone with a ton of triggers if they are already annoyed at you and in a position to launch a strong counterattack.

Second, don't sleep on Athreos's devotion ability. He's always indestructible, sure, but there are certain occasions when you want him to be a creature rather than an enchantment and vice versa. One player might have control magic that can only steal creatures, while another might run Fade into Antiquity to exile enchantments. The high number of sacrifice outlets and cheap creatures in this deck allow you to manipulate Athreos's permanent type on the fly. Because sacrifice outlets can activate at instant speed but only a couple of your permanents have flash, it is generally easier to revert Athreos from a creature to an enchantment than to coalesce his essence into a creature. Bear this in mind.

There's one last piece of noteworthy wisdom. You might be surprised to learn that most people respond to Athreos's triggered ability rigidly. Some players become anxious whenever they are forced to lose life, and don't recognize that card advantage is often the far more powerful resource. These players will almost never pay Athreos's toll. Other players are wary of any sort of graveyard recursion, perhaps to the point of forgetting that their life, while expendable to some degree, is nevertheless keeping them in the game. These players will almost always pay the ferryman. Very few players occupy the wiser middle ground between these two mindsets. Identifying which players fall into which archetypes is a significant part of playing an Athreos deck well.

Boardwipes and Reanimation
Your deck can not only afford to nuke the battlefield, it prefers to do so. That's not to say that you should cast boardwipes indiscriminately, but a well-timed Wrath of God or Damnation will snatch the wind from your enemy's sails and divert it to yours; remember, as all your creatures die, you rack up a tall stack of triggers for Athreos. Never mind the fact that your reanimation spells can easily claim victory from a well-stocked crypt.

Contamination?
Gimping your own mana base might seem counter-intuitive, but you can do a lot more with only black mana than, say, a white-blue-green deck can. Though it can easily cement your victory, be careful! If you cast Contamination when even one opponent can retaliate, your enemies are likely to form a coalition with the sole goal of taking you out. Should you end up wanting other “fun” cards like this one, consider the following:
• Infernal Darkness (It's the same effect, but it requires mana payments instead of creature sacrifices.)
• Mana Web (Lackluster normally, but it combos with Urborg, Tomb of Yawgmoth.)

Darkest Day and Other Combat Cancelers
Light of Day and Darkest Hour totally prevent combat when combined. This can allow you to leisurely build up your resources and bleed out your enemies. Their combo can obviously make you a target, but it can also earn you an ally or two if played judiciously. When and if you wish to attack, just blow up either enchantment. In addition to this combo, you also have Crawlspace, which constrains only combat directed at you. Here are some more cards to strengthen your pillow fort:
• Peacekeeper (Arguably better than this combo, actually. I meant to put him in the list.)
• Silent Arbiter
• Magus of the Moat
• Island Sanctuary (Nobody expects this one, and you have enough card draw to afford its protection.)
• Moat (If you wanna spend $500, lole.)
There are also “Propaganda” effects, which achieve something similar. Such cards didn't quite make the cut in my version of the list, but if you wish to include them, here they are:
• Windborn Muse (Probably the best for your deck 'cause it's a creature.)
• Ghostly Prison
• Norn's Annex
• Sphere of Safety
• Koskun Falls

Leyline of Sanctity and Dealing with Graveyard Hate
Personal hexproof. Sounds nice, right? Actually, the “You have hexproof” ability is a bit of a novice trap, especially in Commander. The most common use for the ability is in sixty-card Constructed, where it waits in sideboards as a foil to burn decks. The thing is, burn decks don't exist in any meaningful number in Commander. There are still plenty of effects that target players in Commander. Just look at Athreos, for example. But more often than not, the benefits of personal hexproof cards simply can't compete against cards that are more useful in more situations.

So why did I put Leyline of Sanctity in your deck? There is one easy answer: graveyard hate. Your deck relies on making the most of death. Creatures die, and you bring 'em back. Should someone exile all the cards from your graveyard, you can't resurrect them. Cards like Tormod's Crypt always say “Exile all cards from target player's library. This is the main reason why I included Leyline of Sanctity; its ability to shield you from other targeted effects is gravy.

It should be noted, however, that cards like Grafdigger's Cage and Rest in Peace are non-targeted effects that are even more dangerous to your deck. In fact, Rest in Piece not only hoses your reanimation effects, it even negates Athreos's signature ability (because it causes creatures to be exiled instead of dying.) You'll need to use removal or non-creature boardwipes to dispatch these threats. Should you end up preferring more removal or non-creature boardwipes in your deck than what is featured in the list, here are a few options:
• Mortify
• Disenchant
• Merciless Eviction
• Cleansing Meditation
There are more besides these; white has a good deal of artifact and enchantment destruction.

New Cards from Ravnica Allegiance?
Ravnica Allegiance's new Afterlife mechanic, though kinda boring, is quite functional and has fair synergy with Athreos. Should you purchase a pack or two, you might find something exciting to add to your deck. Be advised, however: many of the new Orzhov cards, with or without Afterlife, are pretty mediocre.

Sacrifice Effects
The high number of global sacrifice effects are included for nearly the same reason as are the numerous boardwipes; you can afford to lose more resources than most opponents. If you'd like to add more destruction that isn't quite so double-edged, there are plenty of options:
• Attrition
• Font of Agonies
• Big Game Hunter
• Intrepid Hero
• Capashen Unicorn
And so on. Just search Scryfall for cards (preferably creatures so you can reuse them) with removal-relevant text.

Sanguine Bond and Exquisite Blood
Though I didn't include them in your list, it's often said that no Commander deck containing black should be without Sanguine Bond and Exquisite Blood. They're decent cards that form an infinite combo when combined. I personally think they're a bland win condition, but they're a win-con nonetheless, and, in this deck, they pull their weight even when solo.

Reanimation
Your deck contains a lot of grave digging because it's ideally shedding gallons of blood. If you grow fond of such effects and want even more of them, simply use Scryfall's advanced search and look for text that contains “return creature from graveyard.” There are still a few good reanimation cards that aren't in the list, but be advised that too much reanimation creates an overly narrow deck.

Vault of the Archangel
One of the most interesting dynamics of multiplayer formats is how difficult it is to keep track of every threat. Even if each player clearly announces their moves (a scenario that rarely occurs) the sheer number of cards means that even the most perceptive players will occasionally overlook the nail in their coffin. Vault of the Archangel exemplifies this phenomenon. Most players won't pay attention to a nonbasic land that doesn't drastically ramp its controller or possess some other super-splashy effect. If you save its ability until your opponents have forgotten about it, the sudden addition of lifelink and deathtouch to your army can really turn the tables. It's also a decent mana sink in late-game situations when you have an imposing board-state but no spells to cast.

This “surprise you're dead” line of reasoning is also why I included Shizo, Death's Storehouse.