Could you demand that they don't use a particular Arcanum? But "Don't cast that spell" seems limited for tier 3. Thoughts: "Don't cast magic" is presumably more than you can demand of a mage, since that's inherent, specifically called as such in the description of tier 2. If the target fails to resist, any attempt to use the power provokes a clash of wills with the mummy, even if they are no longer present - again, inherent powers, such a werewolf's shapeshifting, cannot be forbidden in this way. Tier 3 (ren, potency 3): The mummy forbids a target from using any power the mummy has previously seen them use. With enough Ba, the mummy can keep this tier going for a number of turns equal to their Ba, unravelling a different enchantment each turn - they also double their ba for the clash of wills. Inherent powers can't be eliminated this way - temporary effects are ended, and cannot be reapplied for Ba + Sekhem turns - permanent effects are suppressed, but will return after that duration. Tier 2 (ba, potency 3): The mummy can detect any magical effects lingering on on a target within their perception - and can eliminate one such effect if they succeed in a clash of wills with its originator. With enough ka, they double their ka for the clash of wills, and can also choose to let through beneficial magic.
Tier 1 (ka, potency 3): The mummy creates a golden-glowing shield around themselves - any magical effect targeting the mummy must first pass a clash of wills - the mummy loses, they can still try to resist the magic again via however that would normally work. With enough Ab, mortal onlookers suffer the extra damage, though it's still only lethal - Inheritors are immune to this, though they must still look away to avoid being blinded. Anyone who fails to resist suffers the blinded Tilt and a point of lethal damage (even recordings of the event can cause this) - ghosts, Lifeless, and undead suffer aggravated damage equal to the successes on a Sekhem roll, collapsing into pillars of salt or ash if killed. Tier 3 (ab, curse, epic): The mummy screams with voice of the Sun, bursting into a blaze of sunlight - every witness to this can see the eye of Re examining them through the mummy's form. Thoughts: This tier isn't tagged epic, so I'm going assume it won't blot out the Sun. With enough Sheut, the mummy can cause nearby flames to merge into a fire they have conjured with the first tier, increasing its intensity. If unleashed along with the first tier, accompanying cultists share the mummy's ability to see in the dark for the rest of the scene, as well as a bonus to stealth rolls. Tier 2 (sheut): The mummy is able to quell any or all light sources they can perceive. With enough Ren, the mummy can make Ren vessels unaffected by the conjured fire, perceive the area around a fire they've made, and also speak through it. The fire lasts for the rest of the story. This does no harm to the cultist, but allows them to set enemies on fire with a grapple, grants a bonus to their initiative, and renders them immune to damage from other fire. The mummy can also conjure this fire within a cultist. They can also use this fire as an attack, setting alight any object of Size equal to Sekhem + Ren or below - this fire is only as intense as a candle, but its size is dependent on the size of the affected object - the fire can only be extinguished by beating it or or fleeing the mummy's presence. They can see in complete darkness, and can conjure a golden flame that burns without heat for the rest of the story, or indefinitely if within their tomb.
Tier 1 (ren): The character's eyes flash with reflected sunlight (even if there isn't any around for them to reflect). People want to focus on their own lives, and deep down, they know the Utterance is not for them. And regular life is still there, demanding to reclaim people's attention. Memories are clouded, conflicted, and people's stories contrict each other - this happens even in real traumas, let alone supernatural ones. Everyone wonder what happened - a disaster? - a terrorist attack of some kind? But it's not repeated. The blood stops raining and is washed away, the afflicted mostly recover. Click to expand.I think there's a certain out of sight out of mind aspect to it.