Why does iPad run out of batteries while it's plugged in? I'm playing Vain Glory with my iPad Pro plugged into the 12W adapter via the supplied Lightning cable. Screen goes black, battery dead. Takes a few minutes to come back on.

Why on earth did the device turn off even though it's plugged into the mains? Why does it show the battery dead symbol, instead of just turning on using the power from the mains to operate? 

This is so frustrating and lame.
The 12.9" iPad Pro can use power faster than the 12W power adapter can supply it. That's why it can run out of power when running for example a game, even though it is connected to the charger.

To avoid that, use the 29W Apple power adapter instead. The 29W adapter is not supplied with the iPad Pro, but must be bought seperately.