To understand the decryptor, one must first understand the encryption engine it attempts to reverse. Ransomware appending the .thundersoft extension is generally classified as a variant of the STOP/Djvu ransomware family.
The "Thundersoft Decryptor" is designed specifically to address this family of malware, relying on the availability of private keys leaked or recovered by security researchers. Thundersoft Decryptor
A file named HOW_TO_DECRYPT.txt or READ_ME_THUNDER.txt appears on the desktop and in every folder containing encrypted files. It typically states: To understand the decryptor, one must first understand
"Your files have been locked by Thundersoft Ransomware. To decrypt them, you must purchase the Thundersoft Decryptor tool for 0.5 Bitcoin. Contact thunder@onionmail.org within 72 hours or the price will double." "Your files have been locked by Thundersoft Ransomware
Emsisoft maintains a collection of free decryptors. Their Thundersoft Decryptor (sometimes labeled under a generic name like "Generic Ransomware Decryptor v2") has successfully recovered data for thousands of users.