Elettra is a 2/2.4 GeV third-generation electron storage ring, located near Trieste, Italy. In view of a substantial increase of the machine performance in terms of brilliance, the so-called Elettra 2.0 upgrade is currently on-going. This upgrade is based on a 6-bends achromat, four dipoles of which having a longitudinally variable field. So far, those dipoles are foreseen to provide a field with a two step profile. The VAriable Dipole for the Elettra Ring (VADER) task, driven by the I.FAST European project, aims at developing a new dipole design based on a trapezoidal shape of the bending radius, which would allow for a further reduction of the horizontal emittance. A prototype of this magnet should be designed my the CIEMAT laboratory and build by KYMA company. This note presents and discusses the new dipole field specification and describes the corresponding optics optimization that was performed in order to reduce at best the emittance of the Elettra ring.It further describes the magnetic and mechanical design, to be the basis for the magnet fabrication.