Modern legal (not sure why this site keeps saying it isn't... looked up every card individually to double check, and Gatherer says they're all Modern legal), but probably not going to be terribly competitive because it's a bit too slow. That being said, it's loads of fun. Load up your opponent with curses; give him/her unenviable choices, like whether to kill the witches or not (Accursed Witch puts another curse on them that makes other curses cost less when it dies, and Bitterheart Witch lets me search my library for a curse and put it directly onto them when it dies); target the curses themselves, or go after my creatures that make curses cost less or let me draw whenever I cast one, or go after me directly; and so on.
Key combos:
The curses. They build on each other. Curse of Exhaustion helps buy you time (along with hand hate, Paths, and Gideon); Curse of Misfortunes lets you search for other curses and put them directly onto the player (key for getting out Overwhelming Splendor); Overwhelming Splendor is the real star, making creatures basically useless and limiting your opponent's ability to affect you by not allowing him/her to activate abilities; Curse of Death's Hold isn't that great on its own, but combined with Overwhelming Splendor, it kills off all your opponent's creatures as a state-based effect; Cruel Reality starts sapping their life by 5 every turn if the previous two curses are out, as your opponent will have no creatures to sacrifice (can't be too many planeswalkers to sacrifice either); Curse of Thirst is another life sapper each turn once other curses are already on the player. Infectious Curse (flip side of Accursed Witch) also could come into play; will make your curses cheaper while also providing you with some life gain.
NOTE: Sideboard is still very much in the works. Suggestions welcome. Rest in Peace and Relic should have obvious uses. Bontu's Last Reckoning works well with curses; since they'll have recurring or static effects every turn without the need for more mana, I can afford to go a turn without untapping. Fragmentize is the cheapest removal that hits the main cards that wreck this deck: Leyline of Sanctity and Witchbane Orb, both 4 CMC. The other slots are definitely still up in the air.
The whole point is to shut your opponent's deck down. Early on, that's accomplished by hand hate and basic creature removal. Turn 3 or 4, your first curse should be coming out -- either Curse of Exhaustion or Curse of Misfortunes. If it's Misfortunes first, it's situational whether you want to get out Curse of Exhaustion first or go ahead and hit them with Overwhelming Splendor. Once you hit turn 5 or 6, your opponent should be having difficulty doing much of anything, with curses combining to shut them out of most of their play options each turn. Once they're shut down, you bring in the curses that start taking life away... or just kill them with whatever creatures you have on your board.
Turn 1 - hand hate (Thoughtseize or Inquisition)
Turn 2 - more hand hate and/or Path a problem creature, or drop a Hero (makes auras cheaper)
Turn 3 - Heliod's Pilgrim (search for an aura), or Gideon, or Curse of Exhaustion (with Hero out)
Turn 4 - Curse of Exhaustion, or Curse of Misfortunes (with Hero), or Accursed Witch (situational)
Turn 5 - Curse of Misfortunes (if already out, search for Overwhelming Splendor and put it on opponent), Bitterheart Witch, Curse of Thirst, or Curse of Death's Hold
Turn 6 - Search for a curse (in order of importance: Overwhelming Splendor, Curse of Death's Hold, Cruel Reality, Curse of Thirst), basically empty your hand at this point.
Turn 7 and on - Kill at your leisure.
Altar's Reap can be used at any point in this process, at instant speed, to sac off a witch to get its effect while also helping you refill your hand. Worst case, if you just really need to draw an answer, you can sac off a Pilgrim, since it's just a 1/2 chump blocker that's already done its job with its ETB... or, if you're REALLY desperate, you could sac a Hero or even a creature-fied Gideon.