Nerva succeded Domitian, brother of Titus and son of Vespasian. Domitian was not a nice person. He was so paranoïd that he had the walls of his apartments covered with a mirror-like stone that allowed him to watch his back at all times.

Unfortunately that was not enough and Domitian was finally terminated by his entourage. It is said that upon seeing his murderers enter his bedroom he reached for a dagger under his pillow but only found the scabbard there. Apparently the plot was well organized but we don't have that many details.

Nerva, an old man already and a well respected senator, was then pulled out of his bed and was volunteered to be the next emperor.

Wisely, he put an end to the purges that had followed Domitian's death, accompanied by the usual "damnatio memoria" voted by those very same senators who had voted praise after praise for the "dominus et deus" Domitian.

He pardoned left and right, so much so in fact that he even retained as Praetorian Prefect one Laelius, who was one of Domitian's creatures. The man knew how to hold a grudge and soon after the praetorians burst into the imperial chambers demanding the heads of Domitian's murderers. So forcefully that Nerva eventually offered his own throat. But the old man was merely pushed aside and Domitian's murderers were murdered in their turn. Adding insult to unjury, the praetorians then forced Nerva to give a speech of thanks.

Nerva knew how to hold a grudge too, so he decided to adopt Trajan, then the very powerful governor of Germany(at least seven legions)as heir and successor.

He sent a message to Trajan with this verse found in Book One of Homer's Iliad. It is known as Chryses' prayer:
"Let your arrows make the Danaeans pay for my tears."

Chryses was a priest of Apollo and Agamemnon had taken his daughter Chryseis captive. When Chryses came to him offering ransom for his daughter, Agamemnon flatly refused, which was a sacrilege against Apollo.

So, Chryses conjured up Apollo Smintheus, the Lord of the Mice, who was the deity supposed to bring plagues upon the world, and asked him to "make the Danaeans pay", which Apollo did by sending a plague upon the Achaeans.

Trajan understood the message perfectly. Soon after he became emperor, even before going to Rome, he called up Laelius and the other conspirators in Germany to "reward" them and upon arrival, they were immediately and terminally "rewarded".
When Trajan arrived in Rome, he was warmly greeted by everybody, including the Praetorians.
Ahh.. Psywar..

Here is the whole prayer from The Iliad: "Hear me, lord of the silver bow who set your power about Chryse and Killa the sacrosanct, who are lord in strength over Tenedos.
O Smintheus, if it ever pleased your heart that I built your temple, if it ever pleased you that I burned all the rich thigh pieces of bulls, of goats, then bring to pass this wish I pray for: let your arrows make the Danaeans pay for my tears."