The Balancer, The Punisher, The Harmer.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Ad-Darr (in Arabic: ٱلْضَّارُ), The One who harms or benefits whoever He wills. He is the one whose wisdom may choose to use forceful corrections to discourage wrongful behavior.
From the root dad-ra-ra (ض ر ر) which has the following classical Arabic connotations: to have an opinion that is opposing, to apply forceful correction, to do an act that is disliked making inconvenient, annoy, distress, to cause adversity, afflict, to harm, injure, and hurt.
The names al-Nafi and Ad-Darr (The one who benefits and The one who harms) are often explained in pairs to acknowledge their opposing nature. To explain each name in isolation is to break the harmony and provide an inaccurate or less comprehensive depiction of who Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is. The same is true of al-Qabid (The Restrictor) and al-Basit (The Expander), al-Awwal (The First) and al-Akhir (The Last).
As an aside, as we've mentioned in previous names, different scholars have different criteria for what qualifies as a name of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. Of the 99 names, 81 are explicitly mentioned in the Qur'an. Therefore, there is not one agreed-upon list for the remaining 18 names. Adh-Darr & An-Nafi are among those names which are not included by certain scholars. This list includes Ibn al-Wazir, Ibn Hazm, and Ibn al-Uthaymeen, among others. However, others, such as Ibn Arabi, Imam al-Bayhaqi, and Imam al-Ghazali, have included these names in their lists.
All things come from Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ, but the proper prescribed Islamic etiquette (Adab - good manners) is to not attribute evil with Allah. Yes, it is Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ who punishes the disobedient, and on the day of judgment, He passes His verdict on whether we will attain paradise. We should be fearful and wish to avoid His punishment. This is part of having taqwa. But let's not forget all His other attributes, how He is al-Hakam (The Impartial Judge) and al-Adl (The Embodiment of Justice). How many of us would want to live in a real anarchist society where there was no law enforcement or no governing authority. That an individual is free to do whatever he/she prefers as long as it does not infringe on another person's right to do what he/she desires.
Fa innahum ‘aduwwwul leee illaa Rabbal ‘aalameen Allazee khalaqanee fa Huwa yahdeen Wallazee Huwa yut’imunee wa yasqeen Wa izaa maridtu fahuwa yashfeen
"Indeed, they are enemies to me, except the Lord of the worlds, Who created me, and He [it is who] guides me. And it is He who feeds me and gives me drink. And when I am ill, it is He who cures me." — (Qur'an 26:77-80)
Note the exemplary manners of Prophet Ibrahim (as) in this ayat. It recognizes Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ as being the cause of everything, Lord of the worlds, the one who creates and gives sustenance. But when it comes to sickness, he refuses to attribute any malice to Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ and instead says, "when I am ill." He then continues recognizing Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ as being the healer of the sickness.
In another example, when Surah Nisa Ayah 4 was revealed to the Prophet Muhammad ﷺ, it worried the sahabah. Abu Bakr asked Prophet Muhammad ﷺ about this verse:
Ayna maa takoonoo yudrikkumul mawtu wa law kuntum fee buroojim mushai yadah; wa in tusibhum hasanatuny yaqooloo haazihee min indil laahi wa in tusibhum saiyi’atuny yaqooloo haazihee min ‘indik; qul kullum min ‘indillaahi famaa lihaaa ‘ulaaa’il qawmi laa yakaadoona yafqahoona hadeesaa
"Wherever you may be, death will overtake you, even if you should be within towers of lofty construction. But if good comes to them, they say, “This is from Allah “; and if evil befalls them, they say, “This is from you.” Say, “All [things] are from Allah.” So what is [the matter] with those people that they can hardly understand any statement?" — (Qur'an 4:123)
It was narrated that Abu Bakr bin Abi Zuhair said: I was told that Abu Bakr said: O Messenger of Allah, how could we be in a good state after this verse: "It will not be in accordance with your desires, nor those of the people of the Scripture, whosoever works evil, will have the recompense thereof` [4:123]? Will we be punished for every bad deed we do? There Messenger of Allah ﷺ said: "May Allah forgive you, O Abu Bakr, do you not fall sick? Do you not get exhausted? Do you not feel sad? Don't calamities befall you?" He said: Of course. He said: "That is the recompense you are given." 
So what we learn from this is Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Ad-Darr (the one who causes harm), but it is never senseless or without benefit. What is best for the believer is to ask for forgiveness and to be steadfast if you're going through a difficult period.
Imam al-Ghazali mentions that anything that happens (good or bad) does not occur without the will of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. The example Imam al-Ghazali gives is to not suppose "poison kills or harms by itself, or that food satisfies or benefits by itself; or that kings or men or Satan, or any creature - be they heavenly bodies or stars or anything else are capable of good or evil, benefit or harm, by themselves." To a person who has the support of al-Nafi (The Benefactor), nothing can hurt them. A clear example of this is fire. It is neither good nor bad, but Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ makes it so. When Nimrod threatened to kill Ibrahim (as) he threw him into the fire, "Allah said, 'O fire, be coolness and safety upon Abraham.' And they intended for him harm, but We made them the greatest losers." (Qur'an 21:69-70)
And so, the believer benefits from knowing these two names of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. We further recognize that it is only Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ who has absolute power to benefit or cause harm. When you're stricken with fear, sickness, or poverty; know that it is from Allah and only he can dispel it. When blessed with happness, wealth, or good health; it is from Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ and only He can sustain it.
We must further reflect upon our lives and ask ourselves what things are causing us harm mentally, spiritually, and physically. Once identified, seek to limit or eliminate those altogether. Alternatively, be conscious and seek things that benefit you. Find ways to incorporate them and make them part of your daily routine. Thank Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ for these blessings as we must recognize they are from Him.
Wa laa tad’u min doonil laahi maa laa yanfa’uka wa laa yadurruk; fa in fa’alta fa innaka izam minaz zaalimeen
"And do not invoke besides Allah that which neither benefits you nor harms you, for if you did, then indeed you would be of the wrongdoers.'" — (Qur'an 10:106)
Qul anad’oo min doonil laahi maa laa yanfa’unaa wa laa yadurrunaa wa nuraddu ‘alaaa a’qaabina ba’da iz hadaanal laahu kallazis tahwat hush Shayaateenu fil ardi hairaana lahooo ashaabuny yad’oo nahooo ilal huda’ tinaa; qul inna hudal laahi huwal hudaa wa umirnaa linuslima li Rabbil ‘aalameen
"Say, “Shall we invoke instead of Allah that which neither benefits us nor harms us and be turned back on our heels after Allah has guided us? [We would then be] like one whom the devils enticed [to wander] upon the earth confused, [while] he has companions inviting him to guidance, [calling], ‘Come to us.’ ” Say, “Indeed, the guidance of Allah is the [only] guidance; and we have been commanded to submit to the Lord of the worlds." — (Qur'an 6:71)
 Sahih bituruqihi wa shawahidihi (Darussalam) Musnad Ahmad 68