Ivypaw is Blackstripe's father- same goes for Thorntail and Thornfang. Blackstripe runs away from his pack because he felt he didn't belong, and Thornfang was forced away after his family was killed (including Thorntail) Before the two were even born, Ivypaw worked under Thorntail as a spy in Thunder Pack in order to take down Bluemoon, their alpha. When the plan eventually fails and Ivypaw's closest friend, Teakfur, dies in battle, Ivypaw gives up his alliance with Thorntail to grieve. When Thorntail sees what Teakfur's passing does to Ivypaw, he starts to feel guilty... finally realizing the effects of war. When he suggest moving on, Ivypaw asks "how?" to which Thorntail responds "family" -- to which he later on takes his own advice; starting his own family and moving past his efforts to start a war with Thunder Pack. The two of them don't talk again after that.

With Blackstripe and Thornfang... Thornfang feigned death to escape Bluemoon; the one who killed Thorntail, his mother, and his sisters. He flees to the Loner forest, where he eventually meets Blackstripe. Blackstripe had only recently run away from his own family, though they were alive, he felt out of place; like he didn't even exist at all, so he left out of spite. When the two meet in the Loner forest, starving and desperate for someone to look to, they promise to stay with one another and start a hunting pact so they can share any food they find in the woods. It's this small alliance that brings them closer together as they grow; a friendship their fathers could've had if Thorntail's lust for blood weren't so strong.