To activate the search effect this card only needs to be sent from the field to the graveyard no matter the reason. So you can use XX-Saber Darksoul as cost for Tribute summons or effects, you can also use him as material for Synchro summoning.
Here are the monsters you can search with XX-Saber Darksoul.
Use these cards to support your X-Saber monsters. Saber Slash destroys opponent's card equal to the number of X-Saber monsters you control. At One With the Sword gives your X-Saber monster a significant boost and allows you to draw when it destroys an opponent's monster but you can only activate it when you control only 1 monster. Sparkling Sword allows you to destroy another card your opponent controls whenever the equipped monster destroyed a monster by battle. Saber Vault gives your X-Saber monsters some boost to their stats depending on their level.