Città delle Ombre

{T}: Sacrificia una delle tue creature ed eliminala dal gioco. Metti un segnalino permanente sui Città delle Ombre.
{T}: Aggiungi X mana incolori all tua riserva, dove X à il numero di segnalino presenti su Città delle Ombre.

City of Shadows

{T}, Exile a creature you control: Put a storage counter on this land.
{T}: Add {C} for each storage counter on this land.

Masters Edition III

{T}, Exile a creature you control: Put a storage counter on City of Shadows.
{T}: Add {X} to your mana pool, where X is the number of storage counters on City of Shadows.

The Dark

oc
T: Sacrifice one of your creatures, but remove it from the game instead of placing it in your graveyard. Put a counter on City of Shadows.
oc
T: Add X colorless mana to your mana pool, where X is the number of counters on City of Shadows.

The storage counters aren't removed when you activate the mana ability.

You can activate City of Shadows's second ability while it has no storage counters on it, though it won't add any mana in this case. You did not tap it for mana, though, in case any abilities (such as the ability of a Fertile Ground enchanting it) care about that.