'Guaranteed' is a bit of a hazy word to use to define anything that follows a Dthrow and here's why: Depending on the action the opponent takes to dodge a Dthrow follow-up, their hitstun amount changes fairly drastically. Let's say you Dthrow Mario at 0%, your hitstun values will look as follows (numbers aren't exact, but very close to it):
+2 Airdodge Advantage
+13 Attack Advantage
+26 Jump Advantage
They can airdodge before you can do anything BUT if started at the exact same time Nair beats every airdodge in the game. If they airdodge just as the hitbox comes out they can avoid you but this requires very good timing, however. Also Peach's airdodge is outlast by Nair's hitbox, so feel free to spam this against her.
They can attack starting from the 13th or so frame but the thing is that since Nair is effectively Marth's Fair range very few characters have a Dair with the right range and speed to beat the Nair (note that special attacks cannot be used at this point). They need to both be longer ranged than Ike's Nair and faster than 10 frames.
Both Fair and Nair are faster than +26 (though Fair is just on the border) so Fair CAN work. Thing is, though, Fair is very much hit or miss in nature. You don't have the 18 frames of active hitbox as you do with Nair, so it's a lot hard to hit. If they go forward and up a bit the best bet is to dash forward a bit and use Aether to hit them. Their airdodge is outlasted, they can't hit you and there is not enough time for them to jump.
Back to lurking.
