Justiciar can be fused with four rares:

Spirithost (drops from 4. Durham Forest)

Heiress (6. Palace of Aravia)

Warmaiden (9. Deadlands)

Elementalist (12. Brimstone Path)

They are all farmable rares that can drop from stages in those particular areas of campaign, or from Mystery/Ancient Shards. Those rares must then be taken up to Level 30 and ascended fully, then you'll be able to fuse them in the Summoning Portal.