Let's just mark this section with a big "DUH." What do you expect from a 2900-line poem that meditates on losing someone to an untimely death? Lots of images relating to graveyards, bones, vaults, etc., that's what.
From the "stones that / Name the under-lying dead" (61-62) to "sepulchral halls" (1118) to "vaults and catacombs" (1120) to numerous mentions of "graves," Tennyson is obsessively interested in the places where the dead end up.
Finally, the poem likens Arthur's remains to a "precious relic" (378), aka the remains of a saint that are worthy to be venerated as a way to be closer to God.