It should be written consensual. It is the third person of the singular of the present indicative or second singular of the imperative of the verb to agree, to agree, to agree, to adopt a common decision. It should be accentuated when the diphthong breaks and forms a hiatus in which the u and the end belong to different syllables. So in this word we have 4 syllables: a-te. nú-a . If we did not carry a tilde in the u we would have 3 syllables: a-te-nua . We would also have 3 syllables if we considered it a sharp word, but then we would write a-te-nuá.