Ok so my old party came across an item that it would create an offspring of the creature/creatures that add their blood to the concoction me and the rest of my party did and I'd like to make this being as a character but what race would it be the races that participated were a lizardfolk, a fallen aasimar, a human, a shadar kai, and an air genasi 1 of the characters is a lycanthrope and another has vampirism so shifter and dhampir are also an option I know I could just do custom lineage but that's the easy way out what race would this character realistically be

I like the thought of shifter, and whenever they transform they just burst into all the other races combined into an amalgam.

Simic Hybrid from the Ravnica setting might be worth a shot. its effectively a mutant style which cna be flavoured quite easily to reflect the "parentage" listed.

I wouldn't say custom lineage is the easy way out.  Shifter seems fine, but I'm unclear as to whether this "blood" construct is intended to be a PC or a monster/NPC.  If it's a PC, you're probably best off doing custom lineage or shifter.  If it's the latter case of a monster or NPC the players will have to ultimately fight or raise or otherwise contend with, well, it's magic dude, so just make a custom monster that has ALL THE FEATURES, and scale its DC accordingly.

There are also a few books on DMsGuild about mixing up lineages in a character, I think the best seller is till "An Elf and an Orc had a Little Baby" ... though I think that work leans a bit too much on presuming binary parental heritage at the expense of more magical or fantastic origins where you could have in this case six or so "parents" (which is not an uncommon trope in fantastic/scifi/superhero tales where the team has to contend with some sort of construction that draws from the whole team).  That said, the tables in Elf and Orc may be really useful if you wanted to invest in it.

Just a thought, but a lot of literature and stories have gone down this road, but with restrictions. In most cases the use of blood from 1 individual, or the combining of blood from 2 individuals seems to be the standard way of things to lessen the difficulty of:  A. coming up with an idea that makes sense. and B. Selling that idea that others can relate with.  So my suggestion is make it a little more simply. You can continue with making a mutant baby from 5 individuals, but I find the concept a bit over my ability to conceptualize. 

I am not saying this is a bad idea, but the easiest way to create writers block is by creating concepts that are bulky or hard to relate to. It is a very easy writing pitfall to fall into!

Have the magical item stolen or lost soon after they start the ritual. They may seek to recover it, but they do not succeed, finding only clues t oa cult and other such foreshadowing. Find a way to get a time jump of some sort in there (either playing for a while longer on other quests, or magic, or time dilation), and you've got the perfect in for a BBEG with all the powers of the party combined and a personal agenda against them. You can look forward to the reveal when you have the BBEG dramatically say "Hello, parents".