Zac is Cain's father.
We do not know who the first man born to Adam was. We only know that Cain is the first one mentioned in the Bible - but this may have been mainly because of Cain's significance as the first murderer.
Grendel is descended from Cain because they are both evil. Cain, brother of Able was the first "evil" person. He killed his brother which led God to banish him, just as Grendel was banished by God. Cain was banished and decided to "build up" his forces such as ogres, elves, phantoms, and giants. Grendel is kind of a giant and ogre combined. In conclusion, Cain is wicked freakin' evil and they wanted to show a connection between him and Grendel to show just how evil Grandel was.
Grendel is compared to Cain in the beginning of the epic because Cain is evil.
According to the novel Grendel, all monsters, including Grendel himself, descend from Cain
Haven Cain's birth name is Haven Jude Cain.
Krisinda Cain's birth name is Krisinda Jeanette Cain.
Ace Cain's birth name is Horace Truman Cain.
Lynn Cain's birth name is Lynn Dwight Cain.
Sam Cain's birth name is Sam Wayne Cain.
Erin Cain's birth name is Erin Elizabeth Cain.
Lamont Cain's birth name is LaMont Terome Cain.