Core
Shoes of Focus
Shoes of Focus are the boots Persephone wants. The cooldown reduction is important overall, and especially early game.
Soul Reaver
Soul Reaver gives you mana, power, and penetration which is all good, but the item's passive is what makes this a core item, as it lets you gouge huge portions of health out of your enemies.
Divine Ruin
Divine Ruin is a core item because it adds more power and penetration, is cheaper than other power-heavy items, and the passive really screws over enemies, even if they are not going for lifesteal.
Secondary Cooldown Item
Persephone wants at least one more cooldown item so she can always be planting more flowers, and getting to use her ult more often is always good. I do not include
Chronos' Pendant because in my experience the cooldown reduction is complete overkill, and it's not worth the trade off of power/defense.
Spear of Desolation
Spear of Desolation is the best option if you are very good at playing Persephone. If you have trouble staying out of the fray or are getting heavily targeted, you probably want one of the following defensive items.
Breastplate of Valor
Breastplate of Valor has very good cooldown reduction and is useful if you are getting ganked by assassins or hunters a lot.
Genji's Guard
Genji's Guard should only be chosen if the enemy team is mostly mages. Otherwise it just is not worth it.
Damage
Rod of Tahuti
Rod of Tahuti gives you a huge burst in power. The passive is nice too, but the power is really the main selling point for Persephone.
Obsidian Shard
Obsidian Shard is great for tank shredding.
Gem of Isolation
Gem of Isolation's passive compliments Persephone's abilities very well. It's also good in team fights.
Doom Orb
Doom Orb could theoretically work, although I have not personally used it myself.
Also not sure how this is being considered a “guide” when it only discusses items.
Just to clarify, is this build for Conquest specifically or all game modes? If it's for anything outside of Conquest I don't think Mage's Blessing is really worth it and I think it would be helpful to specify that.
Regarding the build/items you've listed, I have a few concerns. I don't necessarily think any of the item choices are bad, but rushing Soul Reaver right after boots doesn't scale well since nobody is gonna have much HP or Defense that early into the game, so you're not getting nearly as much from the item as you would building something else and getting Soul Reaver later. I also think that Divine Ruin isn't really a core item. I don't think it's ever bad to build it, but it's still situational and if there's really not much sustain to warrant it, you'd get much more out of either Spear of Desolation or Spear of the Magus in that slot.
I also don't feel like Gem of Isolation is ever really worth it on Persephone. It gives a little bit of lockdown with the slow on a god that doesn't have much chase potential, but in general the item just isn't as strong as it used to be and is very niche, only even considered okay on gods who get extra use from it (ex. Merlin who gets extra damage from enemies who are slowed). And even then, it's not an item you really see picked up because there are just better items for that slot.
My last comment would be the defense options. I feel that Breastplate of Valor and Genji's Guard are only really worth it in a scenario where the enemy team has stacked Magical/Physical backline dive. 99% of the time I feel that Mantle of Discord is just a better, much safer pickup in that item slot and that's if you even need defense at all (which I wanted to note since you have Breastplate in your example build).
Anyways, would love to hear your thoughts and see you expand this into a full guide.
It's more power curve efficient to buy flat pen sooner, so have Divine Ruin or Spear of Desolation listed straight after boots, seeing as they aren't building an early Chronos' Pendant.
Yeah I would never pick Gem of Isolation up on Persephone. It isn't listed in the example build but I'd definitely add a Soul Gem, works really well on her high damage abilities.
Breastplate of Valor works well into a build because of how easily it replaces Chronos' Pendant, I don't feel Genji's Guard really fills that role the same; and of course Mantle of Discord is just better anyway. For a magical prot item Stone of Fal would be the choice but it isn't very good, so maybe an Oni Hunter's Garb for straight tankiness or Bulwark of Hope for survivability.
I think that Genji's Guard would be an okay pickup in a niche situation where you have 4 magicals and the enemy backliner is the only physical. So as an example, let's say the enemy hunter is the only physical and then you have a mage and 3 guardians. The odds of you seeing that are extremely low. BUT in that situation Genji's Guard gives some CDR, especially with the passive, A LOT of MP5 and survivability with health and protections. I think we can both agree though that unless that unlikely scenario happens, you should just opt for a Mantle of Discord.