A biome I've been wanting to return to for a little bit now, the Underdark seemed very apt for this moment as a tie-in to the recent packs for mining assets, dwarven societal assets, as well as a couple of upcoming releases too.
Like in part 2 of the Underdark series, this set omits any attempt to pay due to the many types of peoples that live here and instead is devoted entirely to natural elements (as well as one new rope bridge size and some roughly hewn stone stairs).
Altogether the pack contains a whole new set of mushrooms, crystals, stalagmites, underworld flora, cooling piles of magma, new seamless stone stairs in a variety of paths, and a brand new lava terrain that I personally find to be a big improvement on the prior iteration. And of course, my example map demonstrating an emphasized use of the lava and rock pieces in particular.
The Dungeondraft-ready conversion of Underdark Assets 4 contains all of the standard assets as objects within the pack as well as colorable counterparts of most of the available assets. You'll also find a drawable path for the new rocky/cliffside stairs and terrain for both varieties of lava.
Enjoy!