Ok if my opponent uses premature burial targeting level 6 and I chain mystical does Horus Level 6 still go to the field or not? I know if level 6 is already on the field it stays there but what if I do destroy premature burial when they activate it?

Chain Link 1 - Premature Burial
Chain Link 2 - Mystical Space Typhoon

Chain resolves backwards. MST kills Premature Burial, and Horus never made it to the field, thus, he won't magically come back. Premature Burial (as well as equips and continuous cards) need to be face-up while they resolve in order for their effect to work, as well as remain face-up for their effects to remain.