Retinite is one of the first regular boss you will face in Rookgaard.
Retinite TP is located inside the skeleton archers area.
Once there reach Marrow and follow these directions:
Retinite can combo hard up to 350HP so prepare for a hard battle. He shoots some waves in front, so if possible try to stay diagonal and heal your self when possible to stay full HP most of the time.
The room is really small so it will be difficult to run around and hit him with distance, but in case your main damage is ranged try to round around a little bit you will avoid some hits.
If you have a good melee weapon (e.g: Taurus mace) you will be able to kill him easily, just don't forget to stay diagonal and stay full HP if possible.
For low level rookers you will need around 40-60 pikes to kill 1 Retinite, after some lvls and with good weapons (either distance or melee) you will be able to kill one Retinite with just 10-12 pikes.
Retinite loot may vary, but the most useful stuff is the upgrade crystals that you can use to upgrade your items. It is a rare drop so you will need to hunt him down many times to get one.
Other drops are Platinum Amulet, Black Shield, Crossbow, Sharks (food), platinum coins, bolts, Horn (ring) among other stuff.