Once the half tarrasque becomes level 5,The half tarrasque gains the ability of having advantage on saving throws against spells and other magical effects

The only premade campaign I have is the icespire peak, so i`m guessing that most campaigns only go up too around level 6, so if your dm is using one of the premade campaigns, you probably will not be able too use this ability or the one after it, If someone in the party is a higher level then you, and you are lucky enough too not have a new dm who is using a premade campaign thus allowing you too have level 8 characters, you can decide to Activate this ability, allowing your attacks to increase in damage until you deal at least one more damage point then the strongest characters most damaging attack, at the cost of losing 15 maximum hit points until you disable the ability 

If somehow someone is using magic missile on you even though you level ten, or you are attacked by a a line spell, or a spell that requires a ranged attack roll, you must roll a d6, on a 1-5, nothing abnormal happens, but on a 6, he sends it in the opposite direction, how ever unlike the original tarrasque, rather then acting as though the half tarrasque is the caster, it rather it just sends it from where it originally came from, so if it didn`t come from where the caster was at or the caster moved after casting, they won't get hit