In Crusader Kings you play as a ruler during the crusades. One task you can give your clergy is to Fabricate a Claim on some neighboring county. The implication seems to be that your bishop "researches" your family tree and provides forged evidence that you should be the true ruler of wherever. You can then press your claim, usually with violence.
Is there any historical basis for something like this? How important was having such a claim anyway?
More can always be said, but check this answer from /u/KongChristianV, from the CK 3 AMA:
And this more throught response about medieval rules of war:
I found a good one in Revolt and Resistance in the Ancient Classical World and the Near East (ed. Collins and Manning, 2016), chapter 3, "Revolts in the Assyrian Empire: Succession Wars, Rebellions Against a False King and Independence Movements," by Karen Radner, section 6, "Revolts against a false king."
In that section Radner discusses two examples: first, a revolt during the reign of Assur-dan III during the years 763-759 BCE, which was preceded by an epidemic and then possibly instigated by a total solar eclipse in 763. This was interpreted as a sign that the king had lost the support of the gods and may have provoked insurrections in the cities of Assur, Arrapha, and Guzana.
The second example is more like a Crusader Kings event, this one was an uprising against king Esarhaddon in 670 BC. It started in Harran, where a local woman allegedly experienced an ecstatic revelation, issuing the following prophecy:
"This is the word of the god Nusku: Kingship belongs to Sasi. I shall destroy the name and the seed of Sennacherib!"
(see Luukko and Van Buylaere, 2003: The Political Correspondence of Esarhaddon no. 59 )
Sennacherib was Esarhaddon's father, and Radner speculates that the challenging claimant Sasi might've been a cousin of the royal family, through Sennacherib's father Sargon II.
The insurrection spread fast and even attracted the support of key bureaucrats like Esarhaddon's "Chief Eunuch," Assur-nasir, and Abda, the city overseer of the capital.
Esarhaddon had prior experience dealing with rebel conspiracies however, having previously conducted a mass-execution in 681 of the conspirators who had plotted the murder of Sennacherib, including several of his own brothers. So this time around in 670, he didn't wait for them to act and instead, "the king killed many of his great ones with the sword," according to a Babylonian chronicle's entry for that year.
The disruption was significant enough that the next year, 669, had no "limu-official" chosen to name the year, presumably because too many top officials had been executed in the purge.