i want to make a custum master level boss for my pc but what would be a good amount of xp for him to have to spend and corruption?

What type of boss are you planning to make?

Xenos, Daemon, Mortal, Machine, etc.

The strength of what you plan to make depends on what you're making and what you're making it to fight. Putting something to stand against low level characters, not wise to make something with stats like a bloodthirster. However against high level heretics you would need something with serious bite.

i want to make a chaos space marine boss and the players are low level

Well to be honest there is no official hard and fast rule. You as a GM have a significant amount of (and I'm nicking a term from another user on the FFG boards) GM Handwavium. You dont have to conform to XP limitations or anything like that and if you find something is too strong/weak while its in play, nothing saying you cannot silently adjust the situation for/against the players.

I mean are you wanting something easy, something to give them a moderate challenge, a fight they barely survive by the skin of their teeth, hell do you even go one further and make it one they cant beat and have to run from? We need a little more information than that

A single Chaos Marine would not win against a group of heretics usually due to pure numbers against them. So to put things more in your favour you would need to make a suitably potent Chaos Lord (perhaps consider a few thousand Xp or look at some of the prebuilt character builds in the supplementary books and add a few thousand) but also include lesser minions. The minions would be there to act as a meatshield for the Champion/Lord/boss man and serve as extra firepower.

ok i will try some of that thank you

Your best bet would be to not have a single tough enemy for your heretics to fight. Being alone vs. many usually equals being little to no challenge.

Your best bet would be to not have a single tough enemy for your heretics to fight. Being alone vs. many usually equals being little to no challenge.

Very true... Unless said beastie is really beastly.

One important thing to consider if its a boss type/challenging NPC... Give em Power Weapons or something immune to Power Fields at least for CC... Or you'll soon have an unarmed boss, which kinda makes it impotent.

And if you intend to have it show up singly, consider adding more reactions to it via Step Aside and/or it having a Field Save...

Generally when designing NPCs I always go for what makes sense to me though given that characters background... Since on any given day even the least of NPCs might kill a PC, if the dice gods say so.