Ad Nauseam is a combo deck that wins the game by playing a two-card combo with
Ad Nauseam and either
Angel's Grace or
Phyrexian Unlife. The latter cards do help us not to lose the game for being at negative life total. You then proceed to draw the whole deck with
Ad Nauseam, and pay every cards' casting cost with life so the deck will always put you to a negative life total. From there, there are 2 outs with the deck: exiling three
Simian Spirit Guide and casting
Lightning Storm and discarding sufficient lands to have a lethal damage. They may respond by trying to redirect Lightning Storm or gain some life, but you can simply discard more lands or cast Pact of Negation in response to whatever it is your opponent is trying to resolve. This kill should work most of the time, but won’t work if they have too much life or too many lands. This kill also works at instant speed.
The other win condition is with the new Thassa's Oracle from Theros Beyond Death, you're able to cast it arguably much more easily than Jace, Wielder of Mysteries or Laboratory Maniac since it's less costly. The best thing about the card is you do not need to cantrip or activate any abilities that draw cards, since your win runs off it's ETB effect, and even if it dies in response to the ability resolving, you still win the game on an empty library. So the combo costs UUWB and involves Thassa's Oracle, Spoils of the Vault and Angel's Grace. The idea is that you either hold an Oracle in hand, or have it scryed to the bottom of your library, you cast a Spoils, and in response, resolve an Angel's Grace. from there, you play the fish, scry equal to your devotion to blue, and since your library is empty, you should always win the game.
To this day, Ad Nauseam has always been one of my favourite modern decks, after having tried Restore Balance and Eldrazi & Taxes. It's always been amazing to play out a kill at instant speed, and to stay alive almost indefinitely with the deck's plethora of tools.