How do i make it to where a boss is able to spawn after another has died?

i believe he means after one boss is killed, another is allowed to spawn. similar to the twilight forest mod

Create. Custom Variable called: (YourBossName_Killed), set it on logic, Global Map, Initial Value: False. Create a procedure: When Entity Die:

If entity is type: Custom:YourBoss
   Set VAR:YourBossName_Killed to True

In another procedure where u make ur second boss spawn:

If Get VAR:YourBossName_Killed = true
   Summon: Custom:YourSecondBoss

@ro... how do u do the 

If entity is type: Custom:YourBoss
   Set VAR:YourBossName_Killed to True

and the 

If Get VAR:YourBossName_Killed = true
   Summon: Custom:YourSecondBoss