Tourach's Chant trigger, putting a -1/-1 counter on
Dryad Arbor.
Dryad Arbor dies as a state-based action due to having 0 toughness, triggering its undying ability, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When
Dryad Arbor enters,
Tourach's Chant and
Pandemonium trigger.
Resolve the
Tourach's Chant trigger, putting a -1/-1 counter on