Dam of Llauset.
In the road N230 from Benasque to Vall d'Aran and Vielha you must take the direction to the village of Aneto. After the village the bad road goes to the valley of Llauset. After 9 km we'll arrive to the ruins of a mountain hut, we'll follow to a curious tunnel in Y and we'll turn to the left. After a long and tenebrous tunnel with light we'll arrive to the parking of the dam.
Trailhead: Dam of Llauset (2200m)
Time to summit: 3h 20min
Above the parking we see the entry of a little tunnel, it's the beginning of the path that border the dam in right side to reach the tail of the lake. In this area it's possible to choice both routes from this place. Left, to col of Llauset and right to Ibon of Botornás. The normal route to main peak of Vallibierna goes across the last option (if you climb across col of Llauset, you must to cross the dangerous paso del caballo from Culebres to main summit). A good option is the climb for an route and the descent for the other.
The route of ibon de Botornás goes initially following the good marks of GR-11 (paint red and white) to reach a good free mountain hut (6-7 places).
After the lake we reach a little crossroad but we follow the marks to left side to reach a little col in upper side with a signal. In this place we leave the marks of GR-11 (they continue to col of Vallibierna east) and turn to west (left) signaled as Ibones Chelaus. We follow the cairns (good signaled) to the little Ibones chelaus (ice lakes) and before the estany chelat we turn in W-SW direction to the last ramps of Vallibirna. After a long zig-zag we reach the final ridge (impressive but wide) to the main summit of Vallibierna (3067m)
This is the moment to choice:
a)-the cross of Paso del caballo (very impressive, narrow): generally you must cross like horsing in the ridge or putting the hands in the ridge with the legs on North side. Summit of Tuca de les Culebres (3062m). Descent across the col of Llauset.
b)-descent for the same route of climbing.
In winter crampons and ice-axe.
Rope? I don't know. For the people less experienced if they want to cross the impressive paso del Caballo, it can be considered. You have 2 anchorages, and a metallic ring in the middle of step-crossing.