You will unlock destiny cards for the first time after meeting the Fateweaver on the way to Gohart. As you level you will be given points to spend as you see fit in the three trees: finesse, sorcery and might. The more points you put into each tree, the higher the tiered destiny cards you will unlock. There are single class, dual class, and triple class specializations. Single revolves around putting points in just one tree, dual are a choice of two, and triple is of course all three. The requirements for each card can be found in the parentheses at the top. You must have that many points in that allotted tree to use that specific destiny card. By level 2 you can have the Jack of All Trades destinies (putting one point in each finesse, might and sorcery), by level 5 you can have the dual class card, and by level 37 you will have the top tier card if you put all the rest of your points into only one tree - I believe one more level would net you what you need for top tier if you did the dual or tri-spec.