H5N1 Influenza mutating to facilitate efficient person-to-person transmission.
COVID would be but a mild inconvenience in comparison to a truly pandemic bird flu.
SARS-COV-2 has a case fatality rate less than 2% (possibly up to 10 times less, depending on how many asymptomatic and unreported cases occurred. The "real" CFR may be higher/lower in different ages, countries, health-care environments, etc., but averages across 100M+ reported cases to 2%,
The H5N1 virus which causes "Bird Flu" has had limited transmission from birds to humans thus far, and person-to-person transmission is incredibly rare. However, in individuals who have been infected the fatality rate has been on the order of 60%.
Assume, as with SARS-COV-2, that only the most severe cases have been reported and the real fatality rate is 1/10th the published rate, say 6%. That is still on the order of 30 times the that of covid.
Additionally, while covid was mild-to-asymptomatic in children and young adults, H5N1 show the highest mortality in that group.
See https://www.cdc.gov/flu/avianflu/h5n1-people.htm for more.