Spiky nautilus armor

attaching some spikes on nautilus armor could turn it into weapon

to balance it out, probably only copper, iron and golden armors should be possible to "upgrade" into spiky version, by attaching corresponding metal ingots on them, further balance could be lover protection than non-spiky versions, alternatively only way to get those could be instead rarely on nautilus jockeys, where they would also actively used it to damage player

when nautilus would do "dash" move, with spiky armor would cause some damage depending on speed it hits entity with, instantly stopping nautilus in move, which may be further thing to consider, may be harder to retreat or chase someone with spiky nautilus armor as hitting any mob (be it fish or golem) stops "dash" move, it being trade of

if craftable, spiky armor could break after some amount of attacks, leaving only regular nautilus armor behind, requiring players to unequip that armor and put spikes back again by crafting it with metal ingots/nugets again

mob drops killed by spiky armor would be treated according to who rides nautilus, if no one or mob (regularly drowned) rides nautilus, then it counts as regular mob killed it, if player rides nautilus, then it would use same approach to drops as tamed wolf killing mobs do