(In other words, you can't sacrifice the same creature to satisfy multiple devour abilities.) All creatures devoured this way are sacrificed at the same time
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While you only get to manifest dread once per turn, you can trigger her as many times as you like

Synthoids Aang and Katara Aang and La, Ocean's Fury Aang's Iceberg Aang, A Lot to Learn Aang, Air Nomad Aang, Airbending Master Aang, Destined Savior Aang, Master of Elements Aang, Swift Savior Aang, at the Crossroads Aang, the Last Airbender Aarakocra Sneak Aardvark Sloth Aatchik, Emerald Radian Abaddon the Despoiler Abattoir Ghoul Abbey Gargoyles Abbey Griffin Abbey Matron Abbot of Keral Keep Abdel Adrian, Gorion's Ward Abduction Aberrant Aberrant Manawurm Aberrant Mind Sorcerer Aberrant Researcher Abhorrent Oculus Abhorrent Overlord Abiding Grace Abigale, Eloquent First-Year Abigale, Poet Laureate Aboleth Spawn Abolisher of Bloodlines Abominable Treefolk Abomination Abomination of Gudul Abomination of Llanowar Abomination, Irradiated Brute Abomination, Terrifying Titan Abomination, World Ravager Aboroth Aboshan's Desire Aboshan, Cephalid Emperor Absolute Grace Absolute Law Absolute Virtue Absolver Thrull Absolving Lammasu Absorbing Man Absorbing Man and Titania Abstract Iguanart Abstract Paintmage Abstruse Archaic Abu Ja'far Abuelo, Ancestral Echo Abuna Acolyte Abundance Abundant Growth Abundant Maw Abyssal Gatekeeper

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Tentative Connection)