  Normal type Ia supernovae (SNe) are thought to arise from the thermonuclear
explosion of massive ($>0.8$ M$_\odot$) carbon-oxygen white dwarfs (WDs),
although the exact mechanism is debated. In some models helium accretion onto a
carbon-oxygen (CO) WD from a companion was suggested to dynamically trigger a
detonation of the accreted helium shell. The helium detonation then produces a
shock that after converging on itself close to the core of the CO-WD, triggers
a secondary carbon detonation and gives rise to an energetic explosion.
However, most studies of such scenarios have been done in one or two
dimensions, and/or did not consider self-consistent models for the accretion
and the He-donor. Here we make use of detailed 3D simulation to study the
interaction of a He-rich hybrid $0.69\,\mathrm{M_\odot}$ HeCO WD with a more
massive $0.8\,\mathrm{M_\odot}$ CO~WD. We find that accretion from the hybrid
WD onto the CO~WD gives rise to a helium detonation. However, the helium
detonation does not trigger a carbon detonation in the CO~WD. Instead, the
helium detonation burns through the accretion stream to also burn the helium
shell of the donor hybrid HeCO-WD. The detonation of its massive helium shell
then compresses its CO core, and triggers its detonation and full destruction.
The explosion gives rise to a faint, likely highly reddened transient,
potentially observable by the Vera Rubin survey, and the high-velocity ($\sim
1000\,\mathrm{km s^{-1}}$) ejection of the heated surviving CO~WD companion.
Pending on uncertainties in stellar evolution we estimate the rate of such
transient to be up to $\sim10\%$ of the rate of type Ia SNe.
