I did manage to come across a more elegant solution:
index=some_number source="some_source" "<tag>"
| appendpipe [ stats count
| eval value=case(count==0, "Critical Error") ]
| fields value
| eval range=case(value=="Critical Error", "severe", value!="Critical Error", "low")
Thank you both for your help though!
EDIT: A better answer, due to DalJeanis:
index=some_number source="some_source" "<tag>"
| head 1
| stats count first(value) as value
| eval value=coalesce(value,"Failure")
| fields value
| eval range=if(value=="Failure", "severe", "low")
... View more