Borborygmos and Fblthp

Whenever Borborygmos and Fblthp enters or attacks, draw a card, then you may discard any number of land cards. When you discard one or more cards this way, Borborygmos and Fblthp deals twice that much damage to target creature.
{1}{U}: Put Borborygmos and Fblthp into its owner's library third from the top.

March of the Machine

Whenever Borborygmos and Fblthp enters the battlefield or attacks, draw a card, then you may discard any number of land cards. When you discard one or more cards this way, Borborygmos and Fblthp deals twice that much damage to target creature.
{1}{U}: Put Borborygmos and Fblthp into its owner's library third from the top.

March of the Machine Promos

If there are one or fewer cards in your library, Borborygmos and Fblthp is put on the bottom of your library.

Only the controller of Borborygmos and Fblthp can activate its last ability, and only if it’s on the battlefield.

The first ability of Borborygmos and Fblthp triggers and goes on the stack without a target. If you discard one or more land cards, the second “reflexive” triggered ability will trigger. You’ll choose the target creature for that second ability at that time.

You can’t discard nonland cards, even if you want to.