Prisma came to Android a few months ago with a ton of really impressive photo filters. Whereas some apps with filters try to be subtle, Prisma completely reinterprets the pictures you feed in, turning them into some fancy art. The main drawback; Prisma only worked online. That is finally changing on Android.
Offline mode was announced a while back, but it came to iOS first. Now, the first offline version of Prisma is live in the Play Store. An internet connection is only needed for downloading the styles in Prisma, which I believe happens automatically as you scroll through the list. However, only half of the styles in the app are available in offline more so far. More will come later. Those without offline support will be grayed out when you're not connected.
I have not noticed any substantial difference in speed when running Prisma operations offline. You should have no problem using the app (at least part of it) when offline or if Prisma's servers are down.