The rhythm isn't a call to arms, but a signal that the goblins have made a new enemy.

Kriegskadenz

X{R}: Kreaturen können in diesem Zug nicht blocken, wenn ihr Beherrscher nicht X für jede blockende Kreatur bezahlt.

Cadencia de guerra

Cadence de guerre

Cadenza di Guerra

戦いの連打

Cadência da Guerra

War Cadence

{X}{R}: This turn, creatures can't block unless their controller pays {X} for each blocking creature they control.

Commander 2013

{X}{R}: This turn, creatures can't block unless their controller pays {X} for each blocking creature he or she controls.

Mercadian Masques

o
Xoo
R Creatures can't block this turn unless their controller pays o
X for each blocking creature.

The ability only applies to blocks declared after it resolves. It will not add costs to any blockers already announced.

You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t block unless the controller chooses to pay the cost (a cost of zero is not automatically paid). You can use this to make your creatures unable to block an attacker.