You need what is called "Refined Eiter". What you need to do:
1. Find ancient bones, and look for an ancient bone skull. The Ribcage won't do, 1/2 times there is a skull infront of the ribcage. You need a black iron pickaxe to break the skull and inside where the brain would be, there is a sludge called "Soft Tissue". Collect all of that, and keep gathering more as you progress. (Soft tissue is teleportable)
2. You need to find a dwarven fortress or bunker or camp, in which there would be blue box called "Components". That box contains the needle used to build a sap extractor. If you break the box or anything at all, the drawven camp gets permenantly hostile, which is unfortunate since they have mages and stuff. Mage with one star 2-3 hits you if you have full hp and Anti-magic buffs, so it's safe to say you waren't ever meant to kill them yourself. What you need to do is to wait for Seeker insects to attack the camp, and keep attacking the camp untill they either break the magic blue component box, or kill all the dwarves.
(the magical extractor needle component for the sap extractor is NOT teleportable)
3. then you need to construct a sap collector at a glowing ygdrassil sap, or something like that, then wait for the sap to collect (the sap is also teleportable)
4. You need at least 5 black cores (which are the pruple things you get in the new Mistlands dungeon), in order to build an Eiter refinery. then put the soft tissue and the sap in the refinery, and beware, because that refinery is going to damage buildings around it, so put it somewhere safe away from your main base.
5. Refined either is extremely valuable and used for all sorts of things. Between the weapons and armor + upgrades, I would say you need about 70-80 refined eithers in order for everything, but maybe more wont hurt.

(refined either has no cross mechanics with food you eat to get either)