In case you haven't played him yet, here's how his ult works
It takes the ult of another champ and let you cast it to use it. On champs that can shapeshift, you are allowed to shift into the other form until you die, or Press R again or the timer which is about 5 minutes or so runs out. You can only change into the second form of the champion and not the base form.
For example, against a nidalee, you are allowed to take her cougar transformation skill when she is in both cougar and human form, but you are only able to transform into her cougar form. You will get her Q W E when she is in cougar form as well. This applies to champs like Jayce, elise and Shyvanna.
For champions with stacking mechanics on their ult, as long as you fulfill the requirements, you will be allowed to keep the stacks even after your ult is gone.
For example, Cho gath's ult, you are allowed to gain a stack every time you're able to steal from him and execute a minion/monster/champion/boss with it.
Any ults that requires previous stacks to deal bonus damage will not be available unless that passive stack effect comes from your ult.
If you stole Darius ult, you will get the true damage execution and reset upon kill, but not the bonus stacking from hemorrhage. As long as you can keep executing with darius ult as max rank it's kept as long as you don't die for five minutes.