At the beginning of your draw step, Island Sanctuary causes you to skip your card draw, and prevent creatures without flying and/or islandwalk from attacking you.
Because Mystic Decree causes all creatures to lose flying, only creatures with islandwalk can attack you.

Creatures can't attack you

You do not draw cards during your draw steps

Lock

Island Sanctuary

Mystic Decree

Mystic Decree causes all creatures to lose flying and islandwalk.
Because Stormtide Leviathan prohibits creatures without flying or islandwalk from attacking, no creatures can attack.

Creatures can't attack

Stormtide Leviathan

All creatures lose flying and Islandwalk.
Moat prevents things without flying from attacking.
Creatures can not fulfill the conditions nessasary to attack.

Moat

All creatures lose flying and Islandwalk.
Magus of the Moat prevents things without flying from attacking.
Creatures can not fulfill the conditions nessasary to attack.

Magus of the Moat

Mystic Decree causes all creatures to lose flying.
Form of the Dragon prevents non-flying creatures from attacking you, and since none of your opponent' creatures can have flying, no creatures can attack you.

You can't be attacked

Form of the Dragon

Teferi's Moat prevents creatures of the chosen color without flying from attacking you.
Because Painter's Servant makes all creatures the chosen color, and Mystic Decree causes all creatures to lose flying, no creatures can attack you.

Teferi's Moat

Painter's Servant

Sacrifice Felidar Guardian to Ashnod's Altar to add {C}{C}.
Luminous Broodmoth triggers, returning Felidar Guardian to the battlefield with a flying counter on it.
Felidar Guardian's ETB triggers, exiling Mystic Decree and then returning it to the battlefield.
Repeat.

Infinite colorless mana

Infinite creature ETB

Infinite creature LTB

Infinite death triggers

Infinite creature sacrifice triggers

Luminous Broodmoth

Felidar Guardian

Ashnod's Altar

Sacrifice Felidar Guardian to Phyrexian Altar to add 1 mana of any color.
Luminous Broodmoth triggers, returning Felidar Guardian to the battlefield with a flying counter on it.
Felidar Guardian's ETB triggers, exiling Mystic Decree and then returning it to the battlefield.
Repeat.

Infinite colored mana

Phyrexian Altar

Sacrifice Felidar Guardian to Altar of Dementia to mill target player for 1.
Luminous Broodmoth triggers, returning Felidar Guardian to the battlefield with a flying counter on it.
Felidar Guardian's ETB triggers, exiling Mystic Decree and then returning it to the battlefield.
Repeat.

Combos with "Mystic Decree"