Everything in Thassa's text box is abilities, four of them. However, type changing is handled in layer 4 already, before abilities are removed in layer 6, therefore, depending on how your devotion changes from this point on, Thassa will either be a legendary enchantment with no abilities, or a 5/5 legendary enchantment creature with no abilities, changing between the two as your devotion changes to less than 5 or to 5+.
When Dress Down comes into play, Thassa will be "locked in" as either a creature or not depending on your devotion prior to Dress Down coming into play.
When Dress Down comes into play, Thassa will be "locked in" as either a creature or not depending on your devotion prior to Dress Down coming into play.
No, that's not how it works. In general, as in this case, continuous effects, such as type-changing and ability-removing effects, are "continually and automatically performed by the game" according to the layer system (C.R. 613.5). Thus, Thassa's second ability, as a static ability, will continue to monitor "your devotion to blue" and change its types accordingly, even in the presence of Dress Down, since it has an effect that applies "before" ability-removing effects (C.R. 604.1, 613.1, 613.1d, 613.1f, 613.6, 611.3a). Indeed, that ability's continuous effect is not "locked in", any more than Dress Down's ability-removing effect is (C.R. 604.1, 611.3a). See also this thread.
If I have an Active (devotion 5 or more) Thassa, God of the Sea and someone plays Dress Down...what am I left with?
a 5/5 enchantment creature without indestructible? Or an enchantment without indestructible (no longer a creature)? Or something else?
I guess it boils down to "is the devotion clause an ability that Dress Down wipes away"
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