Vacuumelt depicts some creatures getting unsummoned, which is pretty clear at Vacuumelt. You might not have been able to tell that these creatures are Boros war titans. Check out the inset to see the (slightly warped) Boros symbol on one of their shoulders, and compare with the concept sketch from the Ravnica style guide.
Gruul War Plow -- it's a big, honking plow flanked by all kinds of ragtag Gruul warriors -- check. But did you see who's at the controls? Okay, there are no controls on a Gruul war plow, but did you see who's sitting on top of the thing? It's a certain Borborygmos...
And there's no inset here -- we just wanted you to see Yore-Tiller Nephilim up close and personal. It's a titanic, ancient god from Ravnica's past, and it's made of enormous, petrified chunks of Ravnica itself, great hunks of which crack off as it strides down the citified remnants of its former wilderness home.
But did you notice the grim touch of the bodies hung from cranelike projections on its upper rim? This relates to its reanimation mechanic, sure, but it's also symbolic of its ire. These ancient gods did not return because they smelled the Folger's crystals. They did not return because it's springtime in Ravnica, and their winter's bear-nap is over. No, they returned to express their immortal rage.