Storm of Wars requires at least 1G RAM to run smoothly, unfortunately, iPad 2 only has 512 RAM accordingly to wikipedia. You may experience black screens, unknown errors or significant lags while playing the game on iPad2.

The iPad 2 features an Apple A5 package-on-package (PoP) System-on-chip (SoC), which comprises a 1 GHz dual-core 32-bit Cortex-A9 CPU, 512 MB of RAM.