With the second plotline, you can anticipate the heroes to are unsuccessful to prevent the unsealing, and the sealed evil to be the Final Boss rather of the immediate Big Bad, and for the sealed evil to instantly provide about the Darkest Hour, only to be defeated in any case. Of training course, Evil Is Not a Toy if they triumph, it generally turns out that the Sealed Evil does not care for them (or may have even been manipulating them into liberating them) and disposes of them, generating the Sealed Evil the true Final Boss. The remaining established in the Zendikar block, Rise of the Eldrazi, sees them loosed on the Multiverse. Maybe it couldn't be killed for superior (or much more typically, the circumstances weren't appropriate to deal the ultimate blow), so sealing it was the upcoming greatest point. Another prevalent justification is the Balance Between Good and Evil if they'd killed the historic evil, then a even larger, badder evil would've taken its put, as a result sealing it absent neutralizes the risk but also keeps it in the earth and maintains the harmony.