Aegis Master Feat

Prerequisite: Aegis Expert Feat

• You can select an Aegis effect that costs a spell slot of 2nd level or lower, gaining that feature without expending a spell slot. You can change your selection when you complete a long rest when you prepare your Aegis effects.
• You can use your Aegis feature to cast shield without expending a spell slot an additional time before completing a short rest.

Source: KT Spellblade Class