xubuntu@xubuntu:~$ sudo fsck.ext4 /dev/mapper/luks-79a5cf84-516e-4783-b464-09b00d08a724
e2fsck 1.42.13 (17-May-2015)
ext2fs_open2: Bad magic number in super-block
fsck.ext4: Superblock invalid, trying backup blocks...
fsck.ext4: Bad magic number in super-block while trying to open /dev/mapper/luks-79a5cf84-516e-4783-b464-09b00d08a724

The superblock could not be read or does not describe a valid ext2/ext3/ext4
filesystem.  If the device is valid and it really contains an ext2/ext3/ext4
filesystem (and not swap or ufs or something else), then the superblock
is corrupt, and you might try running e2fsck with an alternate superblock:
    e2fsck -b 8193 <device>
 or
    e2fsck -b 32768 <device>