Pokemon's new battle mechanics each Gen are a case of the good (Megas), the bad (Dynamaxing), and the ugly (Z-Moves). Scarlet & Violet brings with it another new mechanic, Terastalising, which somehow manages to be all three. In some ways, it's the most interesting change to the battle meta yet, with Pokemon able to transform their types to offer new STAB potential (essentially extra power for using a move of your own type), as well as to shield themselves from a weakness. A Fire Pokemon up against a Water-type could Terastalise into a Grass type and flip the battle on its head entirely. But it's not that simple. Nothing about Terastalising is.
Pokemon do not come readily equipped with a funky Terastalising type. All Pokemon can Terastalise, but most will keep their main type. This can still offset any weaknesses introduced by a potential secondary type, but mostly is just another form of Dynamaxing - it makes you stronger. Terastalising lasts for the battle rather than just three turns, but conversely must be recharged manually at a Poke Center rather than being used once per battle as Dynamaxing was. It's the most confusing system yet, but also the most accessible. Megas took a long time to earn in-game, Dynamaxing could only be used in special battles, and Z-Moves... look they were just terrible, okay? I won't entertain any discussion that they might have been decent in any way, shape, or form.
However, not all Pokemon are restricted to their own type. Some can be caught in Tera raids, which can offer a different Tera-type. I currently have a Tera-Electric Hoppip and Tera-Water Toedscool, both of which are very helpful for my Grass gym leader run. Then, later on in the game, you can change a Pokemon's Tera-type anyway, provided you have enough Tera Shards. That makes it by far the most flexible mechanic Pokemon has ever had. Mega was restricted to specific Pokemon, as was Gigantamax, which was like a fancy Dynamax. Though Dynamax was not restricted, where you used it was. Z-Moves, well, again, they don't exist. You have imagined them. They are but a figment of your mind. Expunge them from existence as I have, and be free.
Being able to change a Pokemon's type to give a new sense of depth to your line-up makes this the most interesting mechanic Pokemon has ever had, and with a team of six dual-types, all running a unique Tera-type, it's possible to have all 18 types under your control in a single roster. With all that in mind, you'd have to conclude that this new mechanic is good. But the appliance and long-winded way in which Tera Pokemon can be used and customised is far beyond what these battle gimmicks normally are, and with so much work needed to understand them, you'd have to conclude that the new mechanic is bad. Then looking at the Terastalised Pokemon themselves, which resemble cheap Pandora charm knock-offs, you'd have to conclude that this new mechanic is ugly.
Scarlet & Violet feels like the first game to be influenced by its mechanic, rather than it just being thrown in there afterwards. Dynamax suited the stadium tour vibes of Sword & Shield, but the game could have existed perfectly well without it. Scarlet & Violet's open world is peppered with Tera raids, and only using it once before healing up also feels like a risk/reward balance for a game that asks you to spend as long as you want in the wilderness, but also needs you to hit the various hubs to progress. I'm not sure I like Tera Pokemon, but I damn well admire it, and maybe that's enough. It's just a shame Sun & Moon had no battle gimmick whatsoever.