Plague Beast Lord Mutation Core by Capoc, edited by Xena Dragon
Capoc submitted great spoiler information and pictures. He also posted them on his web site here:

Ah, so you want to dissect a plague beast lord.

First, you must paralyze the creature.

After it is para'd, you have to use a knife or a dagger, and target the lord.

Now, the fun starts, and you need scissors and bandages for this part.
1. This is the circled spot. Double click these to open. If no brain in here, you do not need to bandage.
2. These have x's on them. Use scissors on them to cut them open. If no brain in here, PUT ON A BANDAGE.
3. These have the single red lines on them. Use scissors to cut all veins whose color does not match the pod. In this pic, I have all the veins that should be cut, marked. If, after cutting open and bandaging all these areas, you do not have 4 brains, then you will need a healthy gland to complete the process.

5. (marked as 5 because it isn't always needed)Cut open the organ below the left hand #3(it is purple in this pic, and somewhat squareish, with a green tentacled thing attached)

Remove the diseased gland(green), and place the healthy gland within. This should get you the remaining brain that is needed.

ONCE YOU HAVE ALL FOUR BRAINS: Remove them one by one, placing them into the pan that matches their color. After you rip them out of their pod, you MUST BANDAGE THE BLOOD. Do this to all four brains, and then the center will open, revealing an ice colored brain.

This is the mutation core. Cut it out with scissors, and you're done!

Thanks Capoc! Well done.