Roche's path Edit
If you take Roche's path, you probably won't find out much about this dragon. It appears in the Prologue where it fights against king Foltest, Roche, Geralt, and Triss as they try to make their way to Foltest's children during the siege of La Valette castle. During his interrogation Geralt states this is highly unusual behaviour for a dragon and suggests that its lair was probably located nearby and that it must have felt endangered by the battle. According to Vernon Roche "The ploughing dragon nearly altered the result of the battle." In the end Geralt injures it by trapping its head in a gate and stabbing in the mouth with his silver sword, and the dragon retreats in the general direction of Aedirn.
During the deliberations at Loc Muinne in Chapter III sorceress Síle de Tansarville summons it to her defence and Geralt has to fight it. Síle later states that the dragon is controlled by Philippa Eilhart, another sorceress. The boss fight ends with Geralt flying on the dragon's back and the dragon impaling itself on a broken tree. You can either choose to kill it or to leave it be.
Iorveth's path Edit
You find out a lot more about Saesenthessis if you take Iorveth's path. See Roche's part for happenings during the Prologue. There is one note to which you should pay particular attention: The Crinfrid Reavers mention encountering the dragon on a scouting patrol and Scoia'tael bowing in honor upon seeing it.
In Chapter I listen to the hints regarding Iorveth's motivations. His 'higher goal' will be explained in the next chapter.
In Chapter II you find out this dragon is actually Saskia, the Virgin of Aedirn, as she changes her shape in order to protect Geralt from certain death at the hands of Dethmold. She reveals she is actually daughter of the golden dragon Villentretenmerth also known as Bork Three Jackdaws. Her motivation to unite all the people of Aedirn is explained and it is also stated she's been long preparing the revolt with Iorveth who also invented the Dragonslayer story. Only too late does Geralt find out that as Philippa Eilhart was curing Saskia from poison she also gained control over her using a Rose of remembrance. Philippa means to use her as her safety guard during the deliberations in Loc Muinne and Geralt and Iorveth pursue them there.
In Chapter III you can find counterspell to free Saskia from Philippa's influence. However it can only be done in a cutscene after the boss fight
Saesenthessis appears as a Gwent card, and is a part of the Scoiatel faction.