These drop rates are gathered using this companion application you can run alongside AQW to gather information while you’re playing. These aren’t the actual drop rates and only gathered while farming, hence they are estimations (bit of margin for error). But we tested for long enough periods, so we can say that they are somewhat in this range.

Item Drop Rate Extra Information
Relic of Chaos (/Ultraalteon) 40-50% 300+ kills, not sure of the variance in initial tests.
Relic of Chaos (/Hydrachallenge) 20-25% 300+ kills, 23.57% varied between 20-25. Probably should test with a higher kill count.
Ethereal Essence (/Towerofdoom10) (Shrooms) 60-66%
Ethereal Essence (/Towerofdoom10) (Crickets) 75-80%
Ethereal Essence (/Towerofdoom10) (Dread Thing) 80-85%