The twin dragon gods, Bahamut and Tiamat, are two of the most iconic deities in all of D&D.They gave rise to metallic and chromatic dragons respectively, and yet, like their creations, they are ...

Make Tiamat fall three times -- and inflict enough damage when she does -- and on the third fall she'll fall for good. This will take a very long time, so just avoid her attacks deftly and be patient.