  Intercalation and stacking order modulation are two active ways in
manipulating the interlayer interaction of transition metal dichalcogenides
(TMDCs), which lead to a variety of emergent phases and allow for engineering
material properties. Herein, the growth of Pb intercalated
TMDCs--Pb(Ta$_{1+x}$Se$_2$)$_2$, the first 124-phase, is reported.
Pb(Ta$_{1+x}$Se$_2$)$_2$ exhibits a unique two-step first-order structural
phase transition at around 230 K. The transitions are solely associated with
the stacking degree of freedom, evolving from a high temperature phase with ABC
stacking and symmetry $R3m$ to an intermediate phase with AB stacking and
$P3m1$, and finally to a low temperature phase with again symmetry $R3m$, but
with ACB stacking. Each step involves a rigid slide of building blocks by a
vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the
transitions on warming. At low temperature, bulk superconductivity with
$T_\textrm{c}\approx$ 1.8 K is observed. The underlying physics of the
structural phase transitions are discussed from first-principle calculations.
The symmetry analysis reveals topological nodal lines in the band structure.
Our results demonstrate the possibility to realize higher order metal
intercalated phases of TMDCs, advance our knowledge of polymorphic transitions
and may inspire stacking order engineering in TMDCs and beyond.
