[3] He played a part in the election of antipope Benedict XIII of the Avignon Obedience in 1394, in his status as second most senior cardinal.

According to the narration of events sanctioned by Prignano himself, the "Casus Urbani VI", the Limousin cardinals[15] met immediately after the death of Pope Gregory XI, and decided that their candidate would be Gui de Maillesec.

When opposition to any Limousin papal relative developed, however, they switched their support to Pierre de la Vergne.

On 18 December 1378, Clement assigned Cardinal Guy as Apostolic Nuncio to travel to Flanders, Brabant, Scotland, England and the dioceses of Liège, Utrecht, Cambrai and Tournai, to secure adherence to his papacy;[17] Cardinal Guy departed on 31 December, and is known to have been in Paris at Easter.

[18] On 10 February 1380, the Cardinal received additional powers with respect to England, Scotland, and elsewhere; and on 6 March 1381, these were extended to the diocese of Reims.

Gui was sent to Paris in January 1399, along with Cardinals Pierre de Thury and Amedeo di Saluzzo, to explain the decisions of the Church Council and to seek the assent of King Charles VI of France to the withdrawal of obedience.

[25] The Cardinals were in Paris until the end of June, when the appearance of the plague caused the entire Royal Court to take to the highway.

[26] Other meetings, Councils, and negotiations continued for several years,[27] until finally, on 28 May 1403, a reconciliation and return of France to the Obedience of Benedict XIII was announced.

A major part had been played by Gui, who, along with Amedeo di Soluzzo, had persuaded an assembly of the French clergy on 15 May and had spoken personally in the presence of the King and the Duke of Orleans on 25 May in favor of the reconciliation.

[30] At Livorno, the embassy happened to meet some of the cardinals of Gregory XII who had fled from his court, which was living in exile in Lucca at the time.

[31] On 29 June 1408, the cardinals of both Observances published a document on which they had reached agreement, pledging themselves to summon a General Council of the entire Church, and that if both papal claimants did not give peace to the Church by mutual cessation (resignation), the General Council would take action.

[33] At the fifteenth session,[34] which took place on 5 June 1409, the two papal claimants, Benedict XIII and Gregory XII, were declared to be notorious schismatics, heretics and perjurers, and were anathematized.

The Conclave opened in the Episcopal Palace in Pisa on 15 June, a sort of Novendiales (the traditional nine days of mourning for a dead pope) being observed.

Eventually, on 26 June 1409, the Cardinals agreed unanimously on a Franciscan from Crete who had been raised in Venice, Pietro Filargi, O.Min., who took the throne name Alexander V.[40] Pope Alexander survived his election a little over ten months, dying at Bologna on his way back to Rome on the night of 3–4 May 1410.

8 March] 1411, "laying on my sickbed, and, although weak with old age and unsound of body, healthy in mind, speaking clearly, composed in spirit, constant in faith, by no means doubting in hope, contrite and humble of heart...".

[45] In fact his will was registered with the Parliament of Paris, in accordance with his codicil, on 12 March, and in the document he is spoken of as "deceased" (ledit defunt).