  Our Milky Way (MW) has witnessed a series of major accretion events. One of
the later additions, Gaia-Enceladus, has contributed a considerable mass to the
inner Galaxy, but also generously donated to the outer halo. So far,
associations with present-day MW globular clusters (GCs) have been chiefly
based on their kinematics and ages. Here, we present a chemical abundance study
of the outer halo (R$_{\rm GC}$=18 kpc) GC NGC 1261, which has been suggested
to be an accreted object. We measured 31 species of 29 elements in two stars
from high-resolution Magellan/MIKE spectra and find that the cluster is
moderately metal poor, at [Fe/H]=-1.26. NGC 1261 is moderately
$\alpha$-enhanced to the 0.3-dex level. While from the small sample alone it
remains vague to assert any abundance correlations, the light elements Na,O,Mg,
and Al differ significantly between the two stars, in contrast to the majority
of other elements with smaller scatter, arguing in favour of multiple
generations of stars coexisting in this GC. Intriguingly for its metallicity,
NGC 1261 shows heavy element abundances that are consistent with $r$-process
nucleosynthesis and we discuss their origin in various sites. In particular the
Eu-overabundance quantitatively suggests that one single $r$-process event,
such as a neutron-star neutron-star merger or a rare kind of supernova, can be
responsible for the stellar enhancement or even the cluster's enrichment with
the excess $r$-material. Its heavy element pattern makes NGC 1261 resemble the
moderately enhanced r-I stars that are commonly found in the halo and that have
been detected in Gaia-Enceladus as well. Therefore, combining all kinematical,
age, and chemical evidence we conclude that NGC 1261 is a chemically intriguing
GC that was born in Gaia-Enceladus and has been subsequently accreted into the
MW halo. [abridged]
