so they where the heir to the throne and was raised being told they where going to be the ruler, but they had an older sibling who seized the throne after killing their father(idea is he was a okay ruler but nothing to write home about) and claimed the throne, so they maybe because after being told you are going to be something your entire life and someone then completely takes that away from you it would be very emotionally taxing or maybe another reason they ran away, and ended up taking shelter in a ancient crypt where they met a lich or similar undead and they showed you mercy maybe because you reminded them of a child they lost or again maybe something else they took them in, and raised them as there own and they promised them an empire great enough to crush all of their siblings greatest works, so they taught you to be an emperor and began raising an army of undead to build that empire but alas after many years the undead were discovered, their burgeoning army crushed and they where killed mostly but as the undead handed them an object a ring a book or some other such object as they died that object holds a fragment of the  undead's power and instructions to go and build an empire with what's left of their power. 

this is a dumb idea I have its probably extremely tropey but what do you think

It seems like a solid idea. 
I’m a little confused about how you could have an older sibling and still be the heir. Was the sibling maybe born out of wedlock? That could explain why they were skipped in the line of succession. And fleeing would make a lot of sense for you, as the sibling probably wants to kill you to cement their own grip on power.
And you’d need to work out why you weren’t killed in the raid on your undead patron. Out for supplies? Or they saw a single living person in there with a bunch of undead and thought you were a prisoner, not a willing participant. Or your patron, realizing they were beaten, told you to act like a prisoner and then sealed themself in whatever vessel you have. (I’d probably say it’s the same as whatever you choose for your pact boon.) Your patron, I’d imagine, thinks of this as temporary, and they have plans to cultivate your power until one day they take over your body and live on.

Also, just in the future, please use periods, maybe commas, too. No punctuation makes the post hard to read. 

thanks for the input 

also in terms of succession there are several options including preferenced primogeniture and ultimogeniture 

also yeah I'm sorry about the lack of punctuation I was a hurry when I typed it for some reason

My 2cp worth:

The "Heir to an Empire" thing is something that gets bounced around for a few chracter backstories, main thing with it is to make sure your DM is ok with it and, as a player, don't try to bend it to an advantage beyond what your background may give you..

As a DM I would probably advise a player to reign in the family, don't be heir to a kingdom but try something like heir to a merchant company or small mafia style family as its a lot easier to right plot and adventure hooks featuring those types of things then it is to have adventurers potentially taking on the royal family and their attendant armies and guards as well as bring it to some resolution that would permit the character to continue adventuring, although this is only needed if your DM is planning for the backstories to drive the campaign forward.

Rather than meeting a Lich and being taken in, an army raised and then have it all taken away, I'd leave it as you escaped your siblings purge and hid in a tomb or crypt. It was there that some entity contacted you and promised you power and you agreed and fled to develope it with the hope that you would return one day and have your revenge. Your DM can then use that as a plot hook if they want to, the patron may turn out to be a Lich trapped somewhere under the city, Orcus reaching out to tempt a mortal or maybe just a ghostly ancestor that is angry with the turn of events the sibling has taken by usurping the throne. Leave an element of mystery and something for you to explore.

Quote from Rob76 >>

My 2cp worth:

The "Heir to an Empire" thing is something that gets bounced around for a few character backstories, main thing with it is to make sure your DM is ok with it and, as a player, don't try to bend it to an advantage beyond what your background may give you..

As a DM I would probably advise a player to reign in the family, don't be heir to a kingdom but try something like heir to a merchant company or small mafia style family as its a lot easier to right plot and adventure hooks featuring those types of things then it is to have adventurers potentially taking on the royal family and their attendant armies and guards as well as bring it to some resolution that would permit the character to continue adventuring, although this is only needed if your DM is planning for the backstories to drive the campaign forward.

Rather than meeting a Lich and being taken in, an army raised and then have it all taken away, I'd leave it as you escaped your siblings purge and hid in a tomb or crypt. It was there that some entity contacted you and promised you power and you agreed and fled to develop it with the hope that you would return one day and have your revenge. Your DM can then use that as a plot hook if they want to, the patron may turn out to be a Lich trapped somewhere under the city, Orcus reaching out to tempt a mortal or maybe just a ghostly ancestor that is angry with the turn of events the sibling has taken by usurping the throne. Leave an element of mystery and something for you to explore.

 thanks for the input the idea with the army was the lich started building an army but then adventurers caught wind of a lich raising an army and cleared out the dungeon