Will my iPad get damaged if I connect it to a 18W power bank instead of the 10W charger in the box?
As long as the power bank is a USB standards compliant power bank, then it should not harm your iPad.

Note however that even though a power bank or charger might be able to deliver for example 30W (or even more) - it doesn't mean that the iPad will be able to take advantage of that for faster charging. USB-standards compliant power banks and chargers only deliver higher voltages (and thus effect) when specifically asked for it by the device to be charged. 

For example a 29W Apple charger will be able to fast charge an iPad Pro, but not an ordinary iPad. It will still charge the iPad, but it won't go faster than usual.