There was one important thing that Luctudeus did not tell Hade about the process of forming a soul pact.
The reason why pacts such as the one they wanted to initiate were referred to as 'soul' pacts was because, if it wasn't already clear, souls played a crucial part in the binding process.
Although the concept of souls was a topic not many practitioners were well versed in, much less magic beasts, Luctudeus was an existence who was much, much more knowledgeable than he had made Hade believe.
Souls were the spiritual and immaterial manifestation of every living organism. To establish a connection between two such intangible essences, there needed a medium to act as a bridge and it was this role that had been fulfilled by the blood they exchanged.
However, it was the next part of the procedure that Luctudeus had withheld from Hade.