The mercy seat is the place from which Yahweh spoke to his people in the Old Testament. Exodus 25, verses 17-22.

And thou shalt make a mercy seat of pure gold: two cubits and a half shall be the length thereof, and a cubit and a half the breadth thereof. And thou shalt make two cherubims of gold, of beaten work shalt thou make them, in the two ends of the mercy seat. And make one cherub on the one end, and the other cherub on the other end: even of the mercy seat shall ye make the cherubims on the two ends thereof. And the cherubims shalt stretch forth their wings on high, covering the mercy seat with their wings, and their faces shall look one to another; toward the mercy seat shall the faces of the cherubims be. And thou shalt put the mercy seat above upon the ark; and in the ark thou shalt put the testimony that I shall give to thee. And there I will meet with thee, and I will commune with thee from above the mercy seat, from between the cherubims which are upon the ark on the testimony, of all things which I will give thee in commandment unto the children of Israel.

Also mentioned in Milton's Paradise Lost.

From Book XI:
Praying; for from the mercy-seat above Prevenient grace descending had removed The stony from their hearts, and made new flesh Regenerate grow instead ...
From Book XII:
An ark, and in the ark his testimony, The records of his covenant; over these A mercy-seat of gold, between the wings Of two bright Cherubim; before him burn Seven lamps as in a zodiack representing The heavenly fires...

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