Flashtool 0.9.18.6 is a desktop utility (Windows/macOS/Linux builds exist) used for flashing firmware, kernels, recovery images, and other system partitions to Sony Xperia devices. It supports unlocking bootloaders (where possible), flashing FTF firmware packages, and running lower-level operations via fastboot/flashmode.

While newer versions like 0.9.36.0 exist, version 0.9.18.6 remains a "solid" choice for specific older hardware:

It simplified the process of "bundling" downloaded firmware files into a single, flashable FTF file.

New Flashtool (0.9.19.1) brings compatibility to all Xperia devices

For older Xperias (up to Android 4.4 KitKat):