The attempt at a griffon Joining was made with good intentions, however misguided the attempt was. Griffons swallowing darkspawn blood was a very real danger, and they're as vulnerable to the taint as any other living thing. This was an attempt to keep such griffons alive and healthy, reminiscent of the herbs Ash Warriors used on mabari who contract the taint.
The thing is, griffons can't abide the taint AT ALL, and previous attempts at a Joining have only resulted in griffons killing themselves as they literally tear themselves apart to try and expel the taint from themselves. The blood magic was to get them to accept the taint as just another disease.
It worked, but with unanticipated side effects. The taint progressed much faster in a griffon than a human (shortening thier lives even further than a human) , and granted them vastly greater strength and endurance. It also made them "rabid" They were filled with rage at pretty much everything, even their own riders, and required blood magic to control. As such, the ritual was very rarely used.
Several dozen were made towards the end of the Fourth Blight when things got really bad in the Free Marches. And they were instrumental in bringing down Andoral. But at the same time, their lack of control frakked up the order of battle to begin with. (they were supposed to pull back so ground troops could cover them and help bring down the archdemon. Instead they slipped control and charged headlong, and, though they inflicted grievous injuries n the archdemon, ultimately allowing Garahel to slay it, they all died along with their riders)
After the Blight ended, it was discovered that somehow, the completely psychological sickness the rabid griffons thought they had seemed to become very real, and swept through all the aeries, even those which had no Joined griffons. With these griffons turning on the Wardens, they were forced to be put down. Dooming the griffons to extinction, save for a single clutch which was secretly purified, put into stasis and hidden away,even from the Grey Wardens.