Agrajag is the large, mad, fat, bat-like creature, who was inadvertantly killed by Arthur Dent in every single one of his previous incarnations.
The first time we see Agrajag die, it is in the form of a bowl of petunias and a very surprised sperm whale.
In another incarnation, Agrajag was a rabbit. He described his demise in that incarnation thusly:
"One morning I poked my head for the first time into the bright new world and got it split open by what felt like some primitive instrument made of flint."
That was Arthur, having a meal of rabbit on prehistoric Earth.
After Agrajag became aware of his many, many deaths, he diverted Arthur to a 'Cathedral Of Hate' in order to take his revenge.
He got it wrong, tho, and brought Arthur to the Cathedral before one of his deaths actually occurred. In his anger, he tried to kill Arthur anyway, and wound up dying. Again.