So, I downloaded Dragonborn a while ago but wasn't ready to play it so I just put it off. Immediately after downloading it I was attacked by cultists (I thought in Winterhold, but maybe I am incorrect). I killed them and went on my way. I don't recall looting them. Anyway I played for like 20-30 hours since then, only to find out I was supposed to loot them and grab a letter off them. Now I have the quest subject "Find out who sen the Cultists" but the quest marker send me to a dead cultist near Lakeview Manor with no loot on him besides his clothing. What do I do? Playing this run on Xbox btw.

Just head to Solstheim via the docks in Windhelm.

Ok then what? I've already done quests here, but can't progress the Dragonborne quest.

Go to the Earth Stone near Raven Rock.

I did that, I clicked it and listened to him speak till he started repeating himself. The quest still tells me I need to "figure out who sent the cultists".

The bodies of the first cultists who attack you never de-spawn. You may want to look around and see if you can find the other bodies. One of them should have what you need.

Talk to Neloth, he should be around the stone.