'Need' is a semi-modal verb. This means that it sometimes behaves like an ordinary verb ( as in "He needs to leave") and it sometimes behaves like a modal verb (as in "You needn't worry"). As you probably know, modal verbs, such as 'must', 'may' and so on, don't take a third-person 's' and are followed by the base form of the main verb. In the phrase "Nothing that need concern you" you have an example of 'need' in its modal form. Here's what the Cambridge Dictionary has to say on this: ......................... Affirmatives with the semi-modal need are not common and they are used in formal contexts. There is almost always a negative word (e.g. no one, nobody, nothing) or phrase in the clause, even if the verb phrase is affirmative: No one need think that we are doing this every week. (we are not doing this every week) Nobody need know the name of the person who made the complaint. Not a thing need change on this page. Need comes first in the verb phrase (after the subject and before another verb): Let’s forget about it. No one need know about it. ............................... Look here for more information: https://dictionary.cambridge.org/grammar/british-grammar/need

HI Bunch! "Nothing that need concern you." -->This is common in a spoken language. But to say otherwise in a proper and correct way should be: "No need for you to be concerned!." "Nothing you need to be concerned about." "There's no need for you to be concerned about." "Nothing that concerns you." "Something that should not worry you." There's nothing to worry about!" "There's nothing to be worried about!" Hope this helps! Have a great day!