Sideboard Erebos, God of the Dead and Dark Betrayal if you're concerned about Siege Rhino.

I like this list.

Wondering if Agent of the Fates is better than Mogis's Marauder and Master of the Feast. Does it test well?

If you do decide to include another BB creature, consider Nykthos, Shrine to Nyx. It'll help get your creatures bestowed quickly.

Ok, so, I'll try to keep my suggestions on the budget side of life as i see you've intentionally left out all and any fetch lands Polluted Delta - and Thoughtseize .

That said, your main board has problems. Spiteful Returned has to go. Period. Replacing it with 25cent Gray Merchant of Asphodel is a great idea. Especially to deal with Siege Rhino. 2x Empty the Pits is way overboard. Only 1x if you deem it a necessary win con but you're losing 50cent Murderous Cut as a delve option and amazing control card. You could even drop one for a second 5$ Whip of Erebos. Then Pain Seer, I just don't like it. It's dangerous with the higher CMC cards I'm recommending, but even if you don't take my word on those, I'd still say Sign in Blood is a way better 2 drop for digging cards out and it feeds delve cards faster.

Sideboard problem - Crippling Blight It overlaps with the useful intimidate option on Mogis's Marauder and it overlaps with the hasty response to red decks in Pharika's Cure. I'd drop all 4 Crippling Blight for 1x Whip of Erebos 1x Pharika's Cure 1x Dark Betrayal and 1x Drown in Sorrow