Giving them a light fiend is pretty risky going in blind with fiendsmith around. It's probably better to either dipsea earth and make jord, or if you have an extra name in hand you can make moerk off dark, banish the dipsea, find booty, then make Lys on their turn either with their light or using booty to make something light
Ya as someone else pointed out it reverts to water in the end phase so no worries about a light fiend being left for them. Definitely can be a different type depending on the matchup but blind game one I like making it a light.
Fiendsmith also goes brrrr empty wallet. Plunder is a cheap Rogue deck. You are putting 100+ dollar engine into a deck core that is 20 dollars if that. Just play Snake Eyes Fiendsmith. If you are spending lots of money just go all the way.