1 702.12b A permanent with indestructible canât be destroyed. 2  Such permanents arenât destroyed by lethal damage, and they ignore the state-based action that checks for lethal damage (see rule 704.5g). 3  702.12c Multiple instances of indestructible on the same permanent are redundant.
lethal dose. n. (Pharmacology) the amount of a drug or other agent that if administered to an animal or human will prove fatal.
There are a number of toxic substances, and their toxicity spans a tremendous range. As Quora User mentioned, plutonium is way up there, but it is not the most toxic. Sodium cyanide is reported to be fatal at a dose of 1.5 milligrams per kilogram of body weight. So for an 80 kg person that would be about 120 mg.