Urborg, Tomb of Yawgmoth

All lands are Swamps in addition to their
other types.
"Perhaps I should not hope to convey in mere words
the unutterable hideousness that can dwell in absolute
silence and barren immensity. There was nothing
within hearing, and nothing in sight save a vast
reach of black slime; yet the very completeness of the stillness and the homogeneity of the landscape
oppressed me with a nauseating fear."
-H.P. Lovecraft