The Hail Mary Cloud was, or is, a password guessing botnet, which used a statistical equivalent to brute force password guessing.
The botnet was named and documented by Peter N. M.
[2] The principle is that a botnet can try several thousands of more likely passwords against thousands of hosts, rather than millions of passwords against one host.
Since the attacks were widely distributed, the frequency on a given server was low and was unlikely to trigger alarms.
[2] Moreover, the attacks come from different members of the botnet, thus decreasing the effectiveness of both IP based detection and blocking.