Threat to murder Becket immediately after his trial at Northampton
John of Salisbury, Life of Becket: paragraph 18
Curn autem se in hospitium recepisset, duo magni et fidelissimi proceres ad eum in ipso noctis conticinio accesserunt, vultu miserabiles et lacrimosi, tundentes pectora sua, et confitentes ac protestantes per tremendum judicium Dei, quod indubitanter sciebant viros magnos, et malefactis insignes, ut pote multis pollutos facinoribus, in illius neceт conspirasse, et se ad eum occidendum mutuis invicem astrinxisse juramentis. Ne ergo causa ecclesiae, quae nondum plene innotuerat, in morte ejus pateretur occasum, eadem nocte fugam aggressus est, et uno duntaxat frater sibi ferente solatium, diebus delitescens et noctibus iter peragens, post diem decimum sextum ad portum Sandvici pervenit, et quum potiores vectores non haberet ad manum, in fragili cymbala a duobus sacerdotibus transvectus est in Flandriam, paucis aliis navigium potius impedientibus quam aliquam solatii vel auxilii ferentibus opem.
After, however, he had returned to his lodgings, two great and most faithful nobles came to him that same evening, weeping and with downcast faces. beating their breasts, confessing and protesting, swearing on the terrible judgement of God, that they definitely knew of important men, and [other] evil-minded prominent persons, who were without doubt guilty of many grave and wicked deeds, who were plotting to murder him, and who had combined together having taken reciprocal and mutual oaths with each other to kill him.
Therefore lest the cause of the [Liberty of the] Church, which had not yet become widely known, might be ruined by his death, that same night he set about escaping. And together with only one brother whom he brought with him for comfort, hiding by day and travelling by night eventually he reached the port of Sandwich after sixteen days. And when there were no other better boats for ferrrying to hand, the two priests were borne across the sea to Flanders in a frail skiff [cymba: small rowing boat possibly with a sail] with a few others on board hindering the boat rather than being of any comfort or assistance, or rendering any service.
The same story is described in Thomas Saga Erkibyskups
Eiríkr Magnússon . Thómas Saga Erkibyskups: A Life of Archbishop Thomas Becket in Icelandic. Cambridge University Press. pp. 227–9. ISBN 978-1-108-04921-4.
https://archive.org/stream/thmassagaerkiby01magngoog#page/n253/mode/1up
Late in the evening of that very day there come to the archbishop two men of noble kin and good friends of his, both being in sore tribulation and bringing the tidings from the king's court, that certain persons, right far famed for evil deeds, have allied themselves together so to incur the wrath of God, as to have the life of the archbishop even before their meal the next morning. Lord Thomas then weigheth in his mind this matter, and the danger wherein he now standeth, and wisely considereth, that the cause of the church is yet less known throughout Christendom than it behoveth such a mighty matter to be ; and therefore feareth he, if he suffer death even at this time, that it may be deemed to have come to pass rather through some private guilt of his own, than through his defence of the freedom of God's right; and that the church might reap thereof some detriment rather than any righting. He therefore maketh up his mind, rather to make good an escape, now first for a while, after the example of the blessed apostle Paul, when he fled from Damascus anight-time, for a more enduring profit of holy church.
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