The death of Henry I on 1 December 1135, led to the accession of
Stephen de Blois, to whom Eustace submitted. However, they, just like Eustace, did swear fealty to Stephen after a short time. This capitulation meant that Stephen let them keep the honours and positions they had held under Henry, and Stephen is even found confirming the grants of Eustace's family between 1136 and 1138. Matilda was supported by her uncle King David of Scotland, and he did not accept Stephen's succession peacefully. Thus Eustace was placed in the
front line of a new war. When David invaded northern England, Eustace's castle of Alnwick was among those captured by David in the first two months of the year (though it was returned in March). It has been claimed that Eustace must have gone over to David's side by the end of 1137 when David invaded northern England. There is no proof, however, that Eustace had switched allegiance at this point. After David crossed back into Northumberland in April 1138, Eustace became one of David's active supporters, and during David's siege of
Wark Castle in May, Eustace tried to persuade him to besiege Bamburgh Castle instead. Eustace had had a long association with the Scottish king, or at least with his Norman follower
Robert I de Brus, as Eustace's name appears as a witness to David's charter recording the grant of
Annandale to Robert, issued at
Scone in 1124. Despite the defeat of David, peace the following year brought David victory, his son
Henry becoming Earl of Northumbria and Huntingdon, and under the rule of Earl Henry, Eustace regained many of his Northumberland possessions and received other lands in the earldom of Huntingdon. When a succession dispute for the
bishopric of Durham erupted in 1141, Eustace supported the pro-David
William Cumin against
William de Ste Barbara; and in 1143, Eustace helped negotiate a truce between the two claimants. Eustace's number of known associations with David and Henry after 1144 is small, appearing only as a witness to one charter of Earl Henry issued at
Corbridge at some point between 1150 and 1152. Around 1144 Eustace seems to have entered a beneficial relationship with
Ranulf II, Earl of Chester. Eustace was married to the sister of Ranulf's constable, William fitz William, and in 1143 or 1144 William died. This made Eustace's wife and her sister Matilda joint heiress to the lands and offices of William, who was childless. In either 1144 or 1145, Eustace obtained from Ranulf a large honour with lands mostly in Cheshire, Lancashire and Yorkshire, and gained the office of constable of Chester along with the status as chief counsellor in Ranulf's dominions. Earl Ranulf's patronage also seems to have gained Eustace a grant by
Roger de Mowbray (the earl's captive from the
Battle of Lincoln) of fourteen knight fees worth of estates in Yorkshire and Lincolnshire, with townships along the
river Humber. This was probably part of the attempts of the earl and his half-brother the Earl of Lincoln
William de Roumare to tighten their family's grip on the region. Eustace's position vis-à-vis Stephen probably mirrored that of Ranulf, and like other pro-Matildans there was probably no permanent stabilisation of relations until the settlement between Stephen and Matilda in the winter of 1153. In the following year, Eustace attested a charter King Stephen issued at
York in favour of
Pontefract Priory. ==Death and legacy==