Slimed

Slimed is the not-so scientific term given for both the radioactive quasi-gelatinous bioactive residue left on numerous worlds in this area of space, and the agent causing it. It's similar to a biological silicate hybrid which generates an amorphous radioactive mass with similar molecular analysis to lithium from converted biological mass. It looks like, and is appropriately called, Slime.
The assailants that attacked Home were never effectively contacted, and were eventually given the name Xin. The assault was sudden and nearly complete, killing 72% of the planet's life almost immediately and causing horrid mutagenic effects in everything that... survived. Most of it gratefully died after brief periods.
Since Resoni made the return to interstellar space, it's been found that similar or worse damage happened to several worlds (smaller population colonies seem to have fared worse, often completely wiping them out due to their highly centralized natures). The attacks all ended as quickly as they began.

Type

Physiological