Heaven
TWO DAYS BEFORE THE MASSACRE OF THE THOUSANDS OF ASSASSINS
There was an Assassination Organization called Heaven in the world which few people were aware of its existence.
This organization was considered to be one of the top 10 Assassination Organizations in the world.
The reason for that was because they were big enough and had thousands of assassins to be called an organization with lots of them being top-notch assassins.
But other than having top-notch assassins and lots of members, there was another reason as to why they came to be known as one of the top 10 Assassin Organizations in the world, and that reason was because of a special rule they had, that rule stated if they accept a case of assassination, they would definitely accomplish it and won't stop until their target was eliminated and dead.