This build comes with a focus on Joust and Clash.
Power on Sylvanus works well with the scaling on his abilities at 50% on Wisps and an impressive 125% on Wrath of Terra. The ult can hit a whole team and hit hard, it's just the damage is spread over 5 seconds. It's especially effective on targets you cut the protections of with Verdant Growth.
With these items you get good damage, strong healing and high speed without sacrificing much survivability.
The early mana sustain from Verdant Growth is great, but I'd call Mana Potions a must, especially because this build is otherwise lacking in mana.
Sylvanus can do well with a lot of the starter items. Out of the defensive blessings I prefer Guardian's Blessing for the additional gold towards the luxury items included in this build.
Specialist's Blessing is a great match for Sylvanus too. The mana sustain and movement speed are really useful and this build is otherwise lacking in early cooldown.
Consider Hunter's Blessing for the improved wave/camp clear as it works well with your AoE basic attacks. However, I think the clear is less important in game modes outside of Conquest
Pick up Gauntlet of Thebes early to start stacking an excellent defense item. Hide of the Urchin is a worthy alternative in high action games.
Shoes of the Magi give you a strong start to the scaling of your 2 and ult, with the 10 pen taking the edge off the enemies defenses.
Mantle of Discord brings you up to 120 physical and magical protections in combination with Thebes in addition to the handy 10% cooldown reduction. You'll get plenty of health from other items in the build and the passive is great for Sylvanus who can get a bit stuck in the thick of a fight.
Shield of Regrowth rounds out the health pool with a further 10% CDR. By this point with the passive speed boost your immobile treant will have turned into a speed machine. The total +25HP5 from items is nice too.
The heart of the build: Double Rods
Building both Rod of Asclepius and Rod of Tahuti gives you a high power man on a tree, with both of their passives offering a great boost to healing. Wisps benefits immensely from these items. The health and movement speed on Asclepius are important too.
At full build, level 20 Wrath of Terra is doing 100(+67.5) magical damage per tick, for five ticks.
As one of the downsides of this build is a lack of mana and mana sustain Shoes of Focus are a decent choice. But don't do it. Get pen boots.
Relic Dagger performs much the same role as Shield of Regrowth, but you get your relics up more often instead of having bursts of speed. Winged Blade works well as a replacement if you're getting slowed alot and otherwise dealign with crowd control, but you do give up a bit of %CDR.
Spear of the Magus is a great, cheaper alternative to Rod of Tahuti. Spear provides similar damage as well as pen for other magical damage dealers, but significantly less healing. Avoid picking it up if someone else on your team already has it. If you feel like you could use more survivability, but still want to bruise, you can sacrifice some power and pick up Void Stone instead.
Meditation Cloak adds survivability (especially handy with the increased presence of executes in the game). Rod of Asclepius will boost the healing effect, but you'll actually mostly want med for the mana replenishment with this build. Don't be afraid to use it for mana instead of health.
Blink + ult initiations are a Sylvanus standard. This is my usual level 12 pick up.
Magic Shell surpasses med for general survivability, but I'd prioritise med.
More speed and less slows with Heavenly Wings is usually a good time for Sylvanus and time.