You cast a staff to the ground, which writhes and grows into a giant constrictor snake under your control, which acts on its own initiative count. The snake is friendly to you and your companions. The snake will obey any verbal commands that you issue to it (no action required by you). If you don’t issue any commands, it defends itself from hostile creatures, but otherwise takes no actions. If the snake is reduced to 0 hit points, it dies and reverts to a broken staff. The GM has the creature’s statistics.