Sederunt principes is the proper Introit for the Feast of St. Stephen (December 26); the text comes from various verses (23 and 86, and then Verse 1) of Psalm 119:

Sederunt principes,et adversum me loquebantur; et iniqui persecuti sunt me; adjuva me, Domine Deus meus, quia servus tuus exercebatur in tuis justificationibus. Ps. Beati immaculati in via, qui ambulant in lege Domini. V. Gloria Patri.
Princes sat and spoke against me; and sinners persecuted me: help me, O Lord my God, for thy servant hath practised thy commandments. Ps. Blessed are the undefiled in the way, who walk in the law of the Lord. V. Glory, &c.

A little more about this piece, from Wikipedia:

Pérotin composed organa, the earliest type of polyphonic music; previous European music, such as Gregorian and other types of chant, had been monophonic. He pioneered the styles of organum triplum and organum quadruplum (three and four-part polyphony); in fact his Sederunt principes and Viderunt omnes are among only a few organa quadrupla known.

A prominent feature of his compositional style was to take a simple, well-known melody and stretch it out in time, so that each syllable was hundreds of seconds long, and then use each note of the melody (the tenor, Latin for “holder”, or cantus firmus) as the basis for rhythmically complex, interweaving lines above it. The result was that one or more vocal parts sang free, quickly moving lines (“discants”) over the chant below, which was extended to become a slowly shifting drone.

Another version:

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