The idea of this build is to have your Dark Harvest stacking with you into the late game alongside your passive Absolution. I personally really enjoy playing champions that can scale very well into the late game. Dark Harvest is good on Senna especially because of her ultimate, Dawning Shadow. If a game does end up going late, Dark Harvest adds a lot of execute power to your ult, and generally helps securing kills during teamfights as well. With Senna's huge range, she can stack her Dark Harvest more easily than other champions, because she is safer while doing it.

I choose Cheap Shot instead of Taste of Blood because Senna has good, decent self sustain in the form of her Q, Piercing Darkness. Cheap Shot does a decent amount of damage when you root someone with your W, Last Embrace, though if you feel like you're gonna be up against a poke-heavy lane, Taste of Blood can be better.

Zombie Ward is a really nice rune to run alongside Umbral Glaive. You'll be oneshotting enemy wards left and right, not only denying them vision but taking it for yourself and your team. If I didn't build Umbral Glaive I wouldn't run this rune. However, with Senna having such a low attack speed, Umbral Glaive becomes invaluable. Without it, you'd barely have time to clear more than one single ward during an Oracle Lens.

Ultimate Hunter is my preferred final rune in the last row of Domination. Relentless Hunter can also be taken if you are fond of roaming. I personally prefer Ultimate Hunter. Senna gets a lot of value out of this rune, because her Dawning Shadow CD is very high so the cooldown shaved off with this rune is therefore also high.

Presence of Mind is really good, adding 500 mana to your mana pool, making it harder for you to run out, especially with the 20% you get back on every takedown. The extra 500 mana also increases the AD gained from your Manamune. This rune might take a while to kick in, especially in bad matchups where you can't seem to get a lot of kills. If you feel like this might be the case, Manaflow Band can be a better alternative.

Coup de Grace pairs well with Dark Harvest, allowing you to secure kills more easily with your long range Dawning Shadow or during teamfights.

I rush Manamune because the item just feels so good to play with as Senna. The damage you get is really good especially with a stacked Presence of Mind, and this item helps you negate any potential mana issues from as soon as you start building it at the start of the game. This, in turn, allows you to be more aggressive in lane, get more Piercing Darkness's off, increasing your poke, the amount of Absolution you can stack, as well as your sustain.

I try to rush Boots of Swiftness as fast as I need them. Sometimes this is before I even finish my Manamune. Sometimes it depends on the amount of gold I can go back with. The extra movement speed from these boots is instrumental in being able to safely stack your passive off of the enemies in lane without overextending, getting caught by CC or receiving massive poke damage back every time you try to get a stack. Some matchups I can finish my Manamune easily without even buying Boots. It really depends on the matchup.

After Manamune and Boots of Swiftness, Umbral Glaive is on the menu. You absolutely need this item to clear wards effectively and start stacking those Zombie Wards. The item deals good damage too, so you'll kill two birds with one stone with this item.

After completing your Umbral Glaive you'll be sitting on 4 items. At this point I usually start looking to build an Infinity Edge. With the amount of crit chance you get from Absolution, it would be a waste to not empower those crits with an Infinity Edge. Of course, if the team desperately needs something else, you might want to prioritize survivability or utility over your damage. I have commented on all the potential items you could finish the build with below.

Finish build with whatever you or your team needs.
Lack of sustain? Enemy team is heavy poke? Increase your healing output with an Athene's Unholy Grail.
Enemy stacking armor? Lord Dominik's Regards.
Enemy team has a lot of healing? Mortal Reminder.
Getting focused a lot? Pesky assassins? Guardian Angel.
Armor reduction and some sweet CDR and Phage buff? Black Cleaver.
Just want more damage? Duskblade of Draktharr.

- Edge of Night: This item got nerfed and is not as good as it used to be. However, if you are facing a team where getting caught out by one CC means certain doom, it might be worth picking up. I almost never do. The shield is easy to remove and Senna shouldn't really be near the front line. I advise playing cautiously over getting this item.

- Youmuu's Ghostblade: This item is decent for the movement speed active. The attack speed on it is kind of wasted because of Senna's 0.2x attack speed scaling. During fights, your E already provides you with some movement speed, as well as your passive. This build only runs 2 lethality items, maybe 3 if you really want to push it, but it seems sub-optimal. Duskblade of Draktharr or Black Cleaver is almost always the better pick-up.

- Bloodthirster: Senna already has great self-sustain through her {Piercing Darkness]] healing her whenever she fires it. I have never felt the need for this item. If you need more sustain, Athene's Unholy Grail is the better, more team oriented option. Bloodthirster is expensive as hell, too.

- Rapid Firecannon: Can be useful if you want to poke but really can't afford to get near the enemy. Your {Piercing Darkness]] range scales with your max attack range, so for optimal use of this item do a Q>AA. The attack speed on this item is not very good. You can afford to build one crit item on top of Infinity Edge, otherwise you are wasting too many stats to make the item worth the cost.

- Phantom Dancer: Not a bad item. Attack speed is kind of meh, but the crit gets you to 100% which is nice. The movement speed doesn't hurt either and the shield proc can make a difference. The problem I have with Phantom Dancer is that I consider it a defensive item, and I personally think Guardian Angel is superior. The AD on Guardian Angel is better than the Attspeed and Crit, 40 armour is solid and the passive is better than that of Phantom Dancer.

- Runaan's Hurricane: I've heard it said that this item can be really good into teams with a lot of melee. I personally don't see the appeal. The attack speed is somewhat wasted, the crit and movespeed are okay but not super beneficial. I guess the biggest plus apart from having some AoE damage is that the bolts apply on-hit effects, which includes your passive, Absolution. Maybe grabbing a Runaan's Hurricane early can make a difference in your overall Absolution count, but in this build you'd be grabbing it pretty late and there are almost always better alternatives.

- Statikk Shiv: I guess if you're desperate for waveclear this item can help out. Runaan's Hurricane is probably better.

- Stormrazor: this crit item is more decent than the other ones. It has AD on it, and a low amount of attack speed which means less stat waste on Senna. The slow can be nice for proccing your passive. The only problem with this item is that it's significantly more expensive than other crit items. It's not a bad item to pick up for pure damage, but I personally prefer Duskblade of Draktharr or Black Cleaver over this.

- Trinity Force: no. Senna's base AD doesn't scale, making the Sheen proc very weak. The attack speed is not good, and you can get 20%CDR and Phage effect from Black Cleaver instead. Dodge this item.

- Spear of Shojin: Your ult is on too high a cooldown for this to be effective. Also, your attack speed is low so you wouldn't benefit from the Awakened Dragon passive much. Dodge.

- Essence Reaver: with Manamune and Presence of Mind, you really won't be running into any mana issues. This item fits well into a crit build, but doesn't really give us anything we need.

- Mercurial Scimitar: very very situational, and very expensive too. You might want to consider running Cleanse before the match to save yourself the expense.

- Maw of Malmortius: Not a high prio item. I don't build it a lot. It can be good against 4-5 AP enemies, but in that case you could also be getting MR from Mercury's Treads, and/or Athene's. In case an enemy APC is fed and you want to try to survive the burst, this item is good.

- Blade of the Ruined King: The active can be nice but the sustain is unnecessary, the attack speed on it is meh, and you can't benefit from the %hp damage a lot because of your inherent low attack speed. If tanks are an issue, consider items like a Mortal Reminder, Black Cleaver or Lord Dominik's.

- Frozen Mallet: If your team really has trouble with sticking on targets and enemies keep getting away, this can be a good pickup. Very niche, I don't think I've bought it yet, in any of my Senna games.

- Iceborn Gauntlet: Sheen effect is kind of bad, damage wise. The slow can be nice but it's not consistent, you don't have low-CD abilities to spam. Frozen Mallet would work better, though it is more expensive. Or if you like a slow-based playstyle, you can play Glacial Augment. I'm not a fan.

- Shurelya's Battlesong: a support item, yes! If your team is good in teamfights but lacks proper engage, you can build this item. Pop E, pop Shurelya, and cheese your way into the enemy team with all of your allies charging at them while being untargetable. Situational, but good and fun.

- Redemption: Maybe if you wanted to go full healing supp. Not this build.

- Knight's Vow: I don't recommend it but I won't stop you.

- Locket of the Iron Solari: If your team needs to survive certain AoE, sure? Your ult is better than this item, you probably won't need it.

- Zeke's Convergence: You probably won't be near the enemies for this to be truly useful.

- Mikael's Blessing: sure, if you really need to be able to break a certain CC off your ally. Good supportive item, situationally.

- Twin Shadows: You don't have good AP scalings and this item is nowhere near as good as Athene's to justify building it. I would dodge.