Soul Candle Fusion
Monsters in the City of Aeterna drop Aeterna Candle Tears, which can be used to fuse two Soul Candles into one at Heretic Candlelight - Kandel.
Monsters in the City of Aeterna drop Aeterna Candle Tears, which can be used to fuse two Soul Candles into one at Heretic Candlelight - Kandel.
Fusing Soul Candles with higher item levels costs more Aeterna Candle Tears.
Aeterna Candle Tears
Upgrading Psalms of Aeterna can make the monsters in the City of Aeterna drop Aeterna Candle Tears.
Aeterna Candle Tears cannot be taken out of the City of Aeterna by default. You can save a certain amount of Aeterna Candle Tears for a long time by upgrading Psalms of Aeterna.
When Heretic Candlelight - Kandel appears at the end of the City of Aeterna, you can perform Soul Candle Fusion through her. Keep upgrading Psalms of Aeterna, and the Soul Candle Fusion feature of Heretic Candlelight - Kandel in Town will also be unlocked.

Soul Candle Fusion has three possible outcomes, namely Melted, Shaped, and Mutated.
Melt
When fusing Soul Candles, affixes of identical effects and tiers from the two Soul Candles will be fused into a new affix, and the new affix may be upgraded, downgraded, or remain unchanged.
When both Soul Candles have the same T1 affixes, the fused affix has a chance of evolving into a more powerful T0 affix with one guaranteed negative affix.
Shaping
Once melted, the remaining affixes will either be retained or vanish.
Shaping will pair up the two Soul Candles' Mutated Affixes and keep only one affix for each pair.
Mutated
When being melted, T1 and T0 affixes have a small chance of mutating into special affixes. Mutated affixes are very powerful, however, they usually have serious negative impacts on you.