You need to pick up 100 of these mutant spores.
Remember you can switch shards when you find one and collect it from all shards if its up.
When you have 100, go back to the Vendor and buy the Rat Caller to pop the cheevo.
The vendor also sells Noisome Spores, which you will need for the Kingsward Puzzle.
If you dont want to keep the Rat Caller item then you can sell it straight back to the vendor and get the 100 mutant spores back, you can then use these to buy 8 of the Noisome Spores for the puzzle!
If you do want to keep the rat caller item then you will need to pick up another 80 mutant spores for the puzzle.