Negation map on a one-dimensional complex torus ($z+\Lambda \mapsto -z+\Lambda$)

Negation is a biholomorphic involution of $E=\mathbb{C}/\Lambda$. Its fixed points satisfy $2z\in \Lambda$, so they are indexed by the four elements of $\frac{1}{2}\Lambda /\Lambda$.

Away from the four fixed points, negation acts freely and the quotient map is a holomorphic double covering. Near a fixed point, an invariant coordinate is $w=z^2$, so the quotient extends across the image as a Riemann surface and the map has ramification index two. The Riemann-Hurwitz formula giveshence the quotient has genus zero and is biholomorphic to the Riemann sphere.